US airports, cities on high alert after deadly bombings in Belgium

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U.S. Airports Beef Up Security in Aftermath of Brussels Attacks

NEW YORK (Reuters) -- Airports in major U.S. cities were on high alert on Tuesday, with police out in force after at least 30 people were killed in attacks on a Brussels airport and subway, though officials said there was no evidence of specific threats to the United States.

READ MORE: Delta, United reroute flights from Brussels after blasts

U.S. President Barack Obama and Democratic presidential election contender Hillary Clinton vowed to do more to take on militants, while Republican presidential front-runner Donald Trump called for tighter border security and suggested U.S. intelligence services could use torture to head off future attacks.

More reports from social media about domestic security:

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Brussels Attacks: Domestic security response
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US airports, cities on high alert after deadly bombings in Belgium
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to have been killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to have been killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to be killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
New York Police Department (NYPD) officers stand guard in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
New York Police Department (NYPD) officers stand guard in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
A New York Police Department (NYPD) officer patrols in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
A New York Police Department (NYPD) officer patrols in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
New York Police Department (NYPD) officers patrol in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
New York Police Department (NYPD) officers stand guard in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to be killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
A pair of Metro-North Railroad Police officers patrol in New York's Grand Central Terminal, Tuesday, March 22, 2016. Authorities are increasing security throughout New York City following explosions at the airport and subway system in the Belgian capital of Brussels. (AP Photo/Richard Drew)
Out in full force. Protecting NYC with the #Belgiumattack in mind. Rest assured we are prepared. https://t.co/SRLwsYxihn
#LAXPD has increased patrol in all terminals. Report Suspicious Activity To The Police. https://t.co/aKfOsyY3q3
BREAKING: NYPD to ramp up security at mass transit points, bridges & tunnels, and other landmarks following today's attacks in Brussels.
Full statement on the #Belgium terror attacks. Stepping up NYPD patrols in transit & high profile areas in NYC. https://t.co/BCvrJIH5ZO
Explosions in #Brussels - Always stay alert. If you see something, say something. (888)NYC-SAFE https://t.co/or1qVjY0ZK
NYPD steps up security at @TimesSquareNYC due to Brussels attacks https://t.co/AcKgK8uaaP
DHS will not hesitate to adjust our security posture, as appropriate, to protect the American people. #Brussels
DHS is closely monitoring the unfolding events in #Brussels and we remain in contact with our counterparts in the region.
Heavy police presence and k9 In penn station
Seeing cops in riot gear w/ automatic rifles as I get off at Penn Station. Times like this I hate working close to a landmark. Be safe.
STAY STRONG #BRUSSELS. OUR HEARTS ARE WITH YOU-- https://t.co/yBJ1dGhgiD
Transit Police, including bomb dogs, conducting extra patrols on MAX and transit stations after #Brussels🇧🇪 attack. No known threat to #PDX.
LOS ANGELES, CA - MARCH 22: LAX airport police officers with automatic weapons patrol near the Tom Bradley International Terminal March 22, 2016 in Los Angeles, California. A terrorist attack in Brussels has put law enforcement on high alert in Los Angeles despite no specific threats. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: Los Angeles County sheriff's transit police stand by the entrance to the Metro Red and Purple lines in Los Angeles Tuesday morning, March 22, 2016, as an added measure of security following terrorist attacks in Brussels. (Photo by Irfan Khan/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: LAX airport police officer Cortinas keeps watch at Terminal Two March 22, 2016 in Los Angeles, California. A terrorist attack in Brussels has put law enforcement on high alert in Los Angeles despite no specific threats. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: Los Angeles County sheriff's transit police patrol Union Station in downtown Los Angeles Tuesday morning, March 22, 2016, as an added measure of security following terrorist attacks in Brussels. (Photo by Irfan Khan/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: Los Angeles County sheriff's transit police stand by the entrance to the Metro Red and Purple lines in Los Angeles Tuesday morning, March 22, 2016, as an added measure of security following terrorist attacks in Brussels. (Photo by Irfan Khan/Los Angeles Times via Getty Images)
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The Obama administration was expected to tighten security at U.S. airports following the Brussels airport attack, which occurred in a public hall outside of the security check area.

U.S. Representative Adam Schiff of California, the ranking Democrat on the House intelligence committee, said he was concerned that there was a risk of copy cat attacks and that airport security was still a concern despite changes in recent years.

"Overall though there's a lot more work we still need to do at our airports," Schiff told MSNBC.

RELATED: The devastating scenes in Brussels

77 PHOTOS
Brussels Attacks: Explosions at airport, Maelbeek metro station
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US airports, cities on high alert after deadly bombings in Belgium
People are evacuated from Brussels Airport, in Zaventem, on March 22, 2016. after at least 13 people have been killed by two explosions in the departure hall of Brussels Airport. / AFP / Belga / VIRGINIE LEFOUR / Belgium OUT (Photo credit should read VIRGINIE LEFOUR/AFP/Getty Images)
In this image provided by the Belgian Federal Police in Brussels on Tuesday, March 22, 2016 of three men who are suspected of taking part in the attacks at Belgium's Zaventem Airport. The man at right is still being sought by the police and two others in the photo that the police issued were according to a the Belgian Prosecutors 'probably' suicide bombers. Bombs exploded at the Brussels airport and one of the city's metro stations Tuesday, killing and wounding scores of people, as a European capital was again locked down amid heightened security threats. (Belgian Federal Police via AP)
In this image provided by the Belgian Federal Police in Brussels on Tuesday, March 22, 2016, a man who is suspected of taking part in the attacks at Belgium's Zaventem Airport and is being sought by police. Bombs exploded at the Brussels airport and one of the city's metro stations Tuesday, killing and wounding scores of people, as a European capital was again locked down amid heightened security threats. (Belgian Federal Police via AP)
In this undated combination photo provided by the Belgian Federal Police in Brussels on Monday, March 21, 2016, suspect Najim Laachraoui is shown. Belgian prosecutors appealed to the public Monday for information about Laachraoui who allegedly traveled to Hungary in 2015 with the top suspect in the Paris attacks. The federal prosecutor's office said in a statement they are seeking information about 24-year-old Najim Laachraoui, who is said to have traveled to Syria in February 2013. It said Laachraoui was checked by guards at the Austria-Hungary border while driving in a Mercedes with Salah Abdeslam and one other person. (Belgian Federal Police via AP)
#BREAKING: Two loud explosions at #Zaventem airport in #Brussels https://t.co/JFw9RGLjnh
In this still image taken from video from RTL Belgium people receive treatment in the debris strewn terminal at Brussels Airport, in Brussels after explosions Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (RTL via AP) BELGIUM OUT TV OUT NO ARCHIVE
In this still image taken from video from RTL Belgium people receive treatment in the debris strewn terminal at Brussels Airport, in Brussels after explosions Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (RTL via AP) BELGIUM OUT TV OUT NO ARCHIVE
In this photo provided by Ralph Usbeck an unidentified traveller gets to his feet in a smoke filled terminal at Brussels Airport, in Brussels after explosions Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (Ralph Usbeck via AP)
In this photo provided by Ralph Usbeck a police officers directs passengers in a smoke filled terminal at Brussels Airport, in Brussels after explosions Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (Ralph Usbeck via AP)
This video footage captures the aftermath of the explosions at Zaventem airport, #Brussels https://t.co/wcsNygh91Y
In this photo provided by Georgian Public Broadcaster and photographed by Ketevan Kardava, armed security personnel are seen in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. A developing situation left a number dead in explosions that ripped through the departure hall at Brussels airport Tuesday, police said. All flights were canceled, arriving planes were being diverted and Belgium's terror alert level was raised to maximum, officials said. (Ketevan Kardava/ Georgian Public Broadcaster via AP)
In this photo provided by Georgian Public Broadcaster and photographed by Ketevan Kardava two women wounded in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. A developing situation left at least one person and possibly more dead in explosions that ripped through the departure hall at Brussels airport Tuesday, police said. All flights were canceled, arriving planes were being diverted and Belgium's terror alert level was raised to maximum, officials said. (Ketevan Kardava/ Georgian Public Broadcaster via AP)
In this photo provided by Georgian Public Broadcaster and photographed by Ketevan Kardava a man is wounded in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. A developing situation left at least one person and possibly more dead in explosions that ripped through the departure hall at Brussels airport Tuesday, police said. All flights were canceled, arriving planes were being diverted and Belgium's terror alert level was raised to maximum, officials said. (Ketevan Kardava/ Georgian Public Broadcaster via AP)
This photo provided by Georgian Public Broadcaster and photographed by Ketevan Kardava, shows the scene in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. A developing situation left a number dead in explosions that ripped through the departure hall at Brussels airport Tuesday, police said. All flights were canceled, arriving planes were being diverted and Belgium's terror alert level was raised to maximum, officials said. (Ketevan Kardava/ Georgian Public Broadcaster via AP)
In this photo provided by Ralph Usbeck an unidentified traveller gets to his feet in a smoke filled terminal at Brussels Airport, in Brussels after explosions Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (Ralph Usbeck via AP)
In this photo provided by Georgian Public Broadcaster and photographed by Ketevan Kardava a man is wounded in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. A developing situation left at least one person and possibly more dead in explosions that ripped through the departure hall at Brussels airport Tuesday, police said. All flights were canceled, arriving planes were being diverted and Belgium's terror alert level was raised to maximum, officials said. (Ketevan Kardava/ Georgian Public Broadcaster via AP)
In this photo provided by Georgian Public Broadcaster and photographed by Ketevan Kardava, an injured man lies on the floor in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. A developing situation left a number dead in explosions that ripped through the departure hall at Brussels airport Tuesday, police said. All flights were canceled, arriving planes were being diverted and Belgium's terror alert level was raised to maximum, officials said. (Ketevan Kardava/ Georgian Public Broadcaster via AP)
In this photo provided by Georgian Public Broadcaster and photographed by Ketevan Kardava a man is wounded in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. A developing situation left a number dead in explosions that ripped through the departure hall at Brussels airport Tuesday, police said. All flights were canceled, arriving planes were being diverted and Belgium's terror alert level was raised to maximum, officials said. (Ketevan Kardava/ Georgian Public Broadcaster via AP)
In this photo provided by Ralph Usbeck travellers stand in a smoke filled terminal at Brussels Airport, in Brussels after explosions Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (Ralph Usbeck via AP)
Brussels aftermath video shows survivors huddled together, telling one another: "It's okay". https://t.co/3Y7lEXrrjP https://t.co/KdqFXR9Wft
BRUSSELS, BELGIUM - MARCH 22: Passengers are evacuated from Zaventem Bruxelles International Airport after a terrorist attack on March 22, 2016 in Brussels, Belgium. At least 13 people are though to have been killed after Brussels airport was hit by two explosions whilst a Metro station was also targeted. The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels. (Photo by Sylvain Lefevre/Getty Images)
The blown out windows of Zaventem airport are seen after a deadly attack in Brussels, Belgium, Tuesday, March 22, 2016. Authorities in Europe have tightened security at airports, on subways, at the borders and on city streets after deadly attacks Tuesday on the Brussels airport and its subway system. (AP Photo/Peter Dejong)
A private security guard helps a wounded women outside the Maalbeek metro station in Brussels on March 21, 2016 after a blast at this station located near the EU institutions. Belgian firefighters said at least 26 people had died after 'enormous' blasts rocked Brussels airport and a city metro station today, as Belgium raised its terror threat to the maximum level. / AFP / Michael VILLA (Photo credit should read MICHAEL VILLA/AFP/Getty Images)
People react as they walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium Tuesday March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Geert Vanden Wijngaert)
In this photo provided by David Crunelle, an injured woman leaves the scene at Brussels airport, after explosions rocked the facility in Brussels, Belgium, Tuesday March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing at least 13 people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (David Crunelle via AP) NO SALES - NO ARCHIVES
A picture taken on March 22, 2016 shows a Belgian police vehicle driving past passengers who are evacuating the Brussels Airport of Zaventem. At least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport on March 22, 2016, Belgian media quoted the federal prosecutor as saying. Officials were not immediately available to confirm the figure when contacted by AFP. The federal police earlier confirmed one death. / AFP / Belga / Jonas Roosens / Belgium OUT (Photo credit should read JONAS ROOSENS/AFP/Getty Images)
People react as they walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium Tuesday March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Geert Vanden Wijngaert)
People react outside Brussels airport after explosions rocked the facility in Brussels, Belgium Tuesday March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Geert Vanden Wijngaert)
People react as they walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium Tuesday March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Geert Vanden Wijngaert)
A picture taken on March 22, 2016 shows a Belgian police vehicle driving past passengers who are evacuating the Brussels Airport of Zaventem. At least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport on March 22, 2016, Belgian media quoted the federal prosecutor as saying. Officials were not immediately available to confirm the figure when contacted by AFP. The federal police earlier confirmed one death. / AFP / Belga / Jonas Roosens / Belgium OUT (Photo credit should read JONAS ROOSENS/AFP/Getty Images)
In this image taken from TV people receive treatment as emergency services attend the scene after an explosion in a main metro station in Brussels on Tuesday, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, killing at least 13 people and injuring many others just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo)
In this image taken from TV an armed member of the security forces stands guard as emergency services attend the scene after a explosion in a main metro station in Brussels on Tuesday, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, killing a number of people and injuring many others just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo)
In this image made from video, emergency rescue workers stretcher an unidentified person at the site of an explosion at a metro station in Brussels, Belgium, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said.(APTN via AP)
A picture taken on March 22, 2016 shows smoke rising from the Maalbeek underground, in Brussels, following a blast at the station close to the capital's European quarter. The Brussels metro service was being shut down on March 22, its operator said. The measure came after a rush-hour explosion at Maalbeek station with TV images showing black smoke pouring from the station entrance. / AFP / Belga / Seppe KNAPEN / Belgium OUT (Photo credit should read SEPPE KNAPEN/AFP/Getty Images)
A security perimeter has been set, on March 22, 2016 near Maalbeek metro station in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: Broken glass and blood is seen beside signage outside an entrance to Maelbeek metro station following todays attack on March 22, 2016 in Brussels, Belgium. At least 34 people are thought to have been killed after Brussels airport and a Metro station were targeted by explosions. The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels. (Photo by Carl Court/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: Broken glass is seen outside an entrance to Maelbeek metro station following todays attack on March 22, 2016 in Brussels, Belgium. At least 34 people are thought to have been killed after Brussels airport and a Metro station were targeted by explosions. The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels. (Photo by Carl Court/Getty Images)
Policemen stand guard at the entrance of a security perimeter set near Maalbeek metro station, on March 22, 2016 in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
Rescue teams evacuate wounded people outside the Maalbeek metro station in Brussels on March 22, 2016 after a blast at this station located near the EU institutions. Belgian firefighters said at least 26 people had died after 'enormous' blasts rocked Brussels airport and a city metro station today, as Belgium raised its terror threat to the maximum level. / AFP / - (Photo credit should read -/AFP/Getty Images)
Passengers evacuated after blast at #Maalbeek metro station in #Brussels via @EurActiv https://t.co/O3W8C2rpPm https://t.co/XmIoeKfE9F
In this photo provided by Evan Lamos passengers clamber from a metro carriage after explosions in Brussels Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (Evan Lamos via AP)
In this photo provided by Evan Lamos passengers walk inside the metro system after explosions in Brussels Tuesday, March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (Evan Lamos via AP)
In this image provided by Daniela Schwarzer, smoke is seen at Brussels airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. (Daniela Schwarzer via AP)
Smoke billows from the Zaventem Airport after a controlled explosion in Brussels, Tuesday, March 22, 2016. Bombs struck the Brussels airport and one of the city's metro stations Tuesday, killing and wounding dozens of people, as a European capital was again locked down amid heightened security threats. (AP Photo/Michel Spingler)
People are evacuated from Brussels airport in Zaventem on March 22, 2016 following twin blasts. A string of explosions rocked Brussels airport and a city metro station, killing at least 21 people in apparently coordinated attacks, officials said. == BELGIUM OUT == / AFP / Belga / DIRK WAEM / Belgium OUT (Photo credit should read DIRK WAEM/AFP/Getty Images)
A man with blood stains on his sweater leaves Brussels airport in Zaventem on March 22, 2016 following twin blasts. A string of explosions rocked Brussels airport and a city metro station, killing at least 21 people in apparently coordinated attacks, officials said. == BELGIUM OUT == / AFP / Belga / DIRK WAEM / Belgium OUT (Photo credit should read DIRK WAEM/AFP/Getty Images)
Amateur footage shows scene at #Brussels airport after explosions (video: Bart van Meele) https://t.co/v29k9dvMiM
People stand near Brussels airport after being evacuated following explosions that rocked the facility in Brussels, Belgium, Tuesday March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (AP Photo/Geert Vanden Wijngaert)
Passengers are evacuated from Brussels airport, on March 22, 2016 in Zaventem, after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
A victim is evacuated on a stretcher by emergency services after a explosion in a main metro station in Brussels on Tuesday, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, killing at least 13 people and injuring many others just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Virginia Mayo)
People comfort each other after being evacuated from Brussels airport, after explosions rocked the facility in Brussels, Belgium, Tuesday March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing at least 13 people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (AP Photo/Geert Vanden Wijngaert)
A victim receives first aid by rescuers, on March 22, 2016 near Maalbeek metro station in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
A woman is evacuated in an ambulance by emergency services after a explosion in a main metro station in Brussels on Tuesday, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, killing at least 13 people and injuring many others just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Virginia Mayo)
In this image taken from TV an injured person is evacuated as emergency services attend the scene after an explosion in a main metro station in Brussels on Tuesday, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, killing a number of people and injuring many others just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo)
A woman is evacuated by emergency services after a explosion in a main metro station in Brussels on Tuesday, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, killing at least 13 people and injuring many others just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Virginia Mayo)
People react outside Brussels airport after explosions rocked the facility in Brussels, Belgium Tuesday March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Geert Vanden Wijngaert)
A victim receives first aid by rescuers, on March 22, 2016 near Maalbeek metro station in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
A truck carrying soldiers of the Belgian Army arrives after a explosion in a main metro station in Brussels on Tuesday, March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, killing at least 13 people and injuring many others just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Virginia Mayo)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
A young girl looks out of the window of a bus after being evacuated from Brussels airport, after explosions rocked the facility in Brussels, Belgium, Tuesday March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing at least 13 people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (AP Photo/Geert Vanden Wijngaert)
Passengers leave a gym where they took shelter after the explosions, near the Zaventem Airport in Brussels, Tuesday, March 22, 2016. Bombs struck the Brussels airport and one of the city's metro stations Tuesday, killing and wounding dozens of people, as a European capital was again locked down amid heightened security threats. (AP Photo/Michel Spingler)
Passengers are gathered, on March 22, 2016 near Brussels airport in Zaventem, following its evacuation after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
People react as they walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium Tuesday March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Geert Vanden Wijngaert)
In this photo provided by David Crunelle, injured people leave the scene at Brussels airport, after explosions rocked the facility in Brussels, Belgium, Tuesday March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (David Crunelle via AP) NO SALES - NO ARCHIVES
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
Firefighters arrive at a security perimeter set in the Rue de la Loi near the Maalbeek subway station, in Brussels, on March 22, 2016, after an explosion killed around 10 people, according to spokesman of Brussels' fire brigade A string of explosions rocked Brussels airport and a city metro station on Tuesday, killing at least 13 people, according to media reports, as Belgium raised its terror threat to the maximum level. / AFP / Belga / LAURIE DIEFFEMBACQ / Belgium OUT (Photo credit should read LAURIE DIEFFEMBACQ/AFP/Getty Images)
Passengers who were evacuated from the airport wait in Zaventem, on March 22, 2016, after a string of explosions rocked Brussels airport of Zaventem and a city metro station, killing at least 13 people, according to media reports, as Belgium raised its terror threat to the maximum level. / AFP / Belga / SEPPE KNAPEN / Belgium OUT (Photo credit should read SEPPE KNAPEN/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
People react outside Brussels airport after explosions rocked the facility in Brussels, Belgium Tuesday March 22, 2016. Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said. (AP Photo/Geert Vanden Wijngaert)
People are evacuated from Brussels Airport, in Zaventem, on March 22, 2016. after at least 13 people have been killed by two explosions in the departure hall of Brussels Airport. / AFP / Belga / VIRGINIE LEFOUR / Belgium OUT (Photo credit should read VIRGINIE LEFOUR/AFP/Getty Images)
People stand near Brussels airport after being evacuated following explosions that rocked the facility in Brussels, Belgium, Tuesday March 22, 2016. Authorities locked down the Belgian capital on Tuesday after explosions rocked the Brussels airport and subway system, killing a number of people and injuring many more. Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were. Airports across Europe tightened security. (AP Photo/Geert Vanden Wijngaert)
Passengers gather, on March 22, 2016 near Brussels airport in Zaventem , following its evacuation after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
Passengers leave with their luggages, on March 22, 2016 near Brussels airport in Zaventem , following its evacuation after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
A photo taken on March 22, 2016 shows Brussels Airport, in Zaventem, after two explosions rocked the main hall of the airport. At least 13 people have been killed after two explosions occured in the departure hall of Brussels Airport. Government sources speak of a terrorist attack. The terrorist threat level has been heightened to four across the country. / AFP / BELGA / DIRK WAEM / Belgium OUT (Photo credit should read DIRK WAEM/AFP/Getty Images)
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Islamic State, a militant group that has gained control of large areas of Iraq and Syria and has sympathizers and supporters around the world, claimed responsibility. Amaq, a news agency affiliated to the group, said suicide bombers strapped into explosive belts had staged the attacks on the airport and a rush-hour metro train in the Belgian capital.

Delta Air Lines Inc, United Continental Holdings Inc and American Airlines Group Inc reported that they had canceled or rerouted flights as a result of the attack.

Large numbers of uniformed police officers, some in tactical gear as well as National Guard members in fatigues and carrying long weapons, were on patrol at New York's John F. Kennedy International Airport.

READ MORE: Presidential candidates condemn Brussels attacks

One guard member was overheard telling a colleague "we have to keep an eye out for bags" after reports that many of the wounded at the Brussels airport had severe leg injuries, according to officials, a pattern that suggests an explosion at ground-level, possibly from a bag.

Some travelers in the United States said they hoped airports would further tighten security following the Brussels attack.

"Perhaps it should take place sooner, before you get through security. I find it reassuring," said Mary Ray, 71, a retired government trainer flying home to Manchester, England, from New York after a Caribbean cruise with her husband, Malcolm.

Ray said she is happy to remove her shoes or have her hands swabbed as she passes through airport security: "I think it's a deterrent."

Three U.S. Mormon missionaries were injured in the airport blasts, the Church of Jesus Christ of Latter-Day Saints said.

A witness to Tuesday's attack at the Brussels airport said he heard shouts in Arabic shortly before two explosions ripped through a departure lounge.

READ MORE: #JeSuisBruxelles trends in wake of Brussels bombings to show support for victims of attack

European and U.S. airports have extensive security in place to prevent weapons from being smuggled onto aircraft, but the attack took place in a waiting lounge before screening.

"SCOURGE OF TERRORISM"

Obama addressed the attacks briefly in a speech in Havana on his historic first visit to Cuba, vowing to support Belgium as it seeks out those responsible.

"This is yet another reminder that the world must unite. We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism," Obama said. "We can and we will defeat those who threaten the safety and security of people all around the world."

Islamic State was blamed for killing 130 people in Paris last November. Then in December, a married couple inspired by Islamic State shot dead 14 people in San Bernardino, California.

RELATED: Social media reaction to the Brussels attack

52 PHOTOS
Brussels Attacks: Social media reaction
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US airports, cities on high alert after deadly bombings in Belgium
Do you all remember how beautiful and safe a place Brussels was. Not anymore, it is from a different world! U.S. must be vigilant and smart!
My heart & prayers go out to all of the victims of the terrible #Brussels tragedy. This madness must be stopped, and I will stop it.
Our hearts break for the men and women of Brussels this morning: https://t.co/Z3zl8ArMCG
These terrorists seek to undermine the democratic values that are the foundation of our way of life. They will never succeed. -H
We offer our deepest condolences to the families who lost loved ones in this barbaric attack and to the people of Brussels.
Gov. John Kasich's statement on the terrorist attacks in #Brussels. https://t.co/6gwCCMwJcy
We stand in solidarity with the people of #Brussels, and pray for the victims of this horrendous act of terror.
My thoughts and prayers are with the victims and their families who are impacted by the terrorist attacks in Brussels. #prayforbrussels
Praying for the victims, families and people of #Brussels following today’s brutal terror attacks. We must defeat radical Islamic terrorists
Statement by Attorney General @LorettaLynch on the terror attacks in Brussels https://t.co/s2A5RtUc8t https://t.co/3DsOe9W1pU
My condolences to the victims of this vicious attack along with their families and friends. Our thoughts & prayers are with Belgium today.
My thoughts are with the people of Belgium, and I join our European friends in their determination to combat the threat of terror.
.@JohnKerry was informed this morning of the terrorist bombings in Brussels. He is closely monitoring the situation, extends condolences.
Our thoughts and prayers are with the people of Belgium.
My condolences to the families of those lost today and my prayers go out to all those who have been injured. (1/2)
The US and Europe must be united in striking ISIS wherever they seek safe haven to dismantle this terrorist threat once and for all. (2/2)
We stand with the people of Belgium. These horrible acts of terror and the resulting loss of life remind us we must be vigilant...
May the God of Israel bring peace and comfort to those who mourn from the senseless death and carnage in Brussels this morning.
I strongly condemn these hateful attacks. Our thoughts go out to the victims and their families. We stand united against terrorism.
I am shocked and concerned by the events in Brussels. We will do everything we can to help.
Translation: “Through the #Brussels attacks, the whole of Europe has been hit.”
I'm outraged and deeply saddened by the news that so many have been killed and injured in terrorist attacks targeting the people of Brussels
My thoughts with #Brussels and its citizens after these heinous attacks. Stay in a safe place, follow instructions of authorities
Terror will never win. #brusselsattack
my heart goes out to the people of #Brussels this morning
Woke up to the horrible, tragic news in Brussels. Heartbreaking. Sending much love and many prayers.
Waking up to this heartbreaking news this morning. My thoughts and prayers are with you today, #Brussels. 🙏🏼 https://t.co/pKeYIZf1ok
Good morning. Prayers with Brussels today.
Praying for peace around the world. Brussels my heart breaks for you.
I was supposed to be in the Beautiful city of Brussels soon...My heart is with you all.So terrible https://t.co/9c2SdYioGU
Sympathies, #Brussels. Keep strong & smart. As must we all. 🇧🇪 https://t.co/esLUUjoiMP
Beyond sad and outraged that the scourge of the earth continue to feel the need to kill innocent people everywhere. #Brussels #WeStandWithU
Prayers for #Brussels.
Waking up once again with a heavy heart. Thoughts and prayers for Brussels. https://t.co/KdCpqwZMD2
All we need is love in a time like this❤️ #LoveForAll #Brussels https://t.co/lztM5wMnC1
More violence. More innocent lives lost. More anguish. More anger. Only thing lessening is Hope. Heart goes out to all affected. #Brussels
I am extremely concerned for all caught up in unfolding events in Brussels. Solidarity with emergency services & those affected.
Woke to horrible news in Brussels. My heart and prayers to everyone. Hold your loved ones close every day.
Our thoughts are with the victims of these terrible attacks and with the people of Brussels. We stand with you today and always.
Prayers going out to everyone in Brussels this morning
We're deeply saddened by the news out of #Brussels today. Our thoughts and prayers are with everyone affected.
Thoughts and prayers to all our friends in Brussels. Shocking scenes in such a friendly and fun city.
Terrible acts of terrorism in #Brussels. Stand in solidarity with all those affected.
Thinking of our beautiful fans and everyone in #Brussels . Please be careful. Please. ❤️
My heart goes out to Brussels. What a horrific tragedy. #prayforbrussels
Praying for Brussels
Terrorism is one of the scariest things. It does not function like anything else in this world.… https://t.co/fL96Cevclp
Video shared by @ABconcerts shows some of the solidarity messages in chalk that have appeared in central Brussels https://t.co/xB5WzXxxce
Our hearts are with you, #Brussels. Art by @geesubay https://t.co/9yPaIOdlu2
My heart goes out to all effected by the terrorist attacks in #Brussels. Too much hate and disregard for all life in this world. It's sad
Saddened to wake up to this news. Hard to believe there is so much hate in this world when all you want to believe in is the good.
My heart is broken. These events have eternal impact on the lives of countless people. #Brussels, you're in my prayers
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The attacks in Belgium drew immediate response from candidates seeking their party's nomination to run for the White House in the Nov. 8 election.

Former U.S. Secretary of State Clinton vowed to strengthen her drive to "defeat terrorism and radical jihadism." She said on CNN that the United States must do more to coordinate intelligence with Europe.

Billionaire Trump told NBC's "Today" program: "If they could expand the laws, I would do a lot more than waterboarding. You have to get the information from these people."

He was referring to the practice of pouring water over someone's face to simulate drowning as an interrogation tactic. Obama banned its use by U.S. interrogators after taking office in 2009.

Officials in cities including New York, Washington, Boston and Miami said they were aware of no specific threats targeting them following the Brussels attack.

READ MORE: Belgian flag to light up Eiffel Tower after Brussels attacks

While the attack immediately sparked discussion of further strengthening airport security, some pointed out the inherent difficulty of securing spaces that offer broad public access.

"There are limits to exactly how exhaustive those perimeters can become," U.S. Secretary of State John Kerry said in an interview with Telemundo. "So people need to be vigilant, everybody needs to take precautions. I think it underscores to everyone the importance of people uniting."

As she rushed to meet clients arriving at New York's JFK airport, 31-year-old Shannon Freyer worried that adding further layers of security at airports, which have already imposed extensive restrictions since the Sept. 11, 2001 hijacked plane attacks, would do little to make people safer.

"It's just more time involved," Freyer said. "They started making us take off our shoes years ago and they still haven't stopped a thing."

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