Belgium bombings raise security alerts at US airports

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Denver Airport Partially Evacuated Amid Fears of Brussels-Style Attack

NEW YORK, March 22 (Reuters) -

Major U.S. transportation hubs were placed on alert on Tuesday and Denver International Airport briefly evacuated part of its main terminal in a false alarm there hours after suicide bombings in Brussels killed at least 30 people.

Despite public safety concerns unleashed by the violence in Belgium's capital, U.S. Homeland Security Secretary Jeh Johnson said the agency had no intelligence that would point to a similar attack being plotted against the United States.

Makeshift memorial in Brusselsafter attacks

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Memorials in Brussels after attacks
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Belgium bombings raise security alerts at US airports
People holding a banner reading "I am Brussels" behind flowers and candles to mourn for the victims at Place de la Bourse in the center of Brussels, Tuesday, March 22, 2016. 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. (AP Photo/Martin Meissner)
Young women hold each other at a makeshift memorial in front of the stock exchange at the Place de la Bourse (Beursplein) in Brussels on March 22, 2016, following triple bomb attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. A series of explosions claimed by the Islamic State group ripped through Brussels airport and a metro train on March 22, killing around 35 people in the latest attacks to bring bloody carnage to the heart of Europe. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
People bring flowers and candles to mourn at the Place de la Bourse in the center of Brussels, Tuesday, March 22, 2016. 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. (AP Photo/Martin Meissner)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
TOPSHOT - A photo taken on March 22, 2016 shows a sign peace made of candles in front of the Bourse of Brussels in tribute to the victims of Brussels following triple bomb attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. Belgium launched a huge manhunt on March 22 after a series of bombings claimed by the Islamic State group ripped through Brussels airport and a metro train, killing around 35 people in the latest attack to bring carnage to the heart of Europe. / AFP / BELGA / Aurore Belot / Belgium OUT (Photo credit should read AURORE BELOT/AFP/Getty Images)
TOPSHOT - People light candles in tribute to victims at a makeshift memorial in front of the stock exchange at the Place de la Bourse (Beursplein) in Brussels on March 22, 2016, following triple bomb attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. A series of explosions claimed by the Islamic State group ripped through Brussels airport and a metro train on March 22, killing around 35 people in the latest attacks to bring bloody carnage to the heart of Europe. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: A young girl lights a candle at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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)
TOPSHOT - People stand hand in hand in tribute to victims at a makeshift memorial in front of the stock exchange at the Place de la Bourse (Beursplein) in Brussels on March 22, 2016, following triple bomb attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. A series of explosions claimed by the Islamic State group ripped through Brussels airport and a metro train on March 22, killing around 35 people in the latest attacks to bring bloody carnage to the heart of Europe. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather to leave tributes at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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)
A message written on the ground reads 'Brussels is beautiful' next to flowers and candles following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People gather at Place de la Bourse in support of the victims after the terrorist attacks on March 22, 2016 in Brussels, Belgium. At least 31 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 Aurelien Meunier/Getty Images)
People bring flowers and candles to mourn at the Place de la Bourse in the center of Brussels, Tuesday, March 22, 2016. 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. (AP Photo/Martin Meissner)
BRUSSELS, BELGIUM - MARCH 22: People light candles at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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: People hold up a banner as a mark of solidarity at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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: Candles are lit at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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)
People bring flowers and candles to mourn for the victims at Place de la Bourse in the center of Brussels, Tuesday, March 22, 2016. 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. (AP Photo/Martin Meissner)
People mourn for the victims at Place de la Bourse in the center of Brussels, Tuesday, March 22, 2016. 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. (AP Photo/Martin Meissner)
A placard reads 'Share your love' at a makeshift memorial following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
Writing on the asphalt reads "More stars in Brussel's heaven" at Place de la Bourse in the center of Brussels, where people write hundreds of messages on the ground to remember the victims of Tuesday's attack, Tuesday, March 22, 2016. 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. (AP Photo/Martin Meissner)
A woman reads messages written on the ground at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
People walk over messages written on the ground at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
People leave candles and flowers in tribute to victims of triple bomb attacks in front of the stock exchange building in the city center of Brussels on March 22, 2016. A series of apparently coordinated explosions ripped through Brussels airport and a metro train, killing at least 26 people in the latest attacks to target Europe. / AFP / Belga / NICOLAS MAETERLINCK / Belgium OUT (Photo credit should read NICOLAS MAETERLINCK/AFP/Getty Images)
People leave messages and flowers in tribute to victims of triple bomb attacks in front of the stock exchange building in the city center of Brussels on March 22, 2016. A series of apparently coordinated explosions ripped through Brussels airport and a metro train, killing at least 26 people in the latest attacks to target Europe. / AFP / Belga / NICOLAS MAETERLINCK / Belgium OUT (Photo credit should read NICOLAS MAETERLINCK/AFP/Getty Images)
People gather around a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
A placard reads share your love next to flowers and candles laid on the ground following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
A man looks at flowers and messages outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. At least 26 people were reported dead. (AP Photo/Geert Vanden Wijngaert)
A woman places candles in the shape of a heart outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. At least 26 people were reported dead. (AP Photo/Geert Vanden Wijngaert)
Solidarity messages are written in chalk outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. At least 26 people were reported dead. (AP Photo/Geert Vanden Wijngaert)
A heart in the colours of the Belgian flag is taped onto a bag as people gather around a makeshift memorial following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
People write messages on the ground at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
A man writes the word 'Peace' in different languages at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
Two people write solidarity messages in chalk outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. At least 26 people were reported dead. (AP Photo/Geert Vanden Wijngaert)
A man walks by solidarity messages written in chalk outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. At least 26 people were reported dead. (AP Photo/Geert Vanden Wijngaert)
A replica of the Manneken Pis statue stands at a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
People hold hands as they gather at a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
People hold hands as they gather at a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
People light candles at a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
A replica of the Manneken Pis statue is placed among candles at a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: People walk across slogans written on the ground in chalk saying 'Brussels, Belgium, Today' at Beursplein sqaure following todays attack on March 22, 2016 in Brussels, Belgium. At least 34 people are thought to have been killed after Brussels airport and the 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. on March 22, 2016 in Brussels, Belgium. (Photo by Alexander Koerner/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: A painting is left as a tribute at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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: A message is written on a wall following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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: Flowers are placed into the mouth of a lion statue as people gather to leave tributes at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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: A candle is lit as people gather to leave tributes at the Place de la Bourse following today's attacks on March 22, 2016 in Brussels, Belgium. At least 31 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)
A man lights a candle next to the iconic statue of Manneken Pis outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. (AP Photo/Geert Vanden Wijngaert)
A memorial to attack victims with a Belgian flag and flowers is set up outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. (AP Photo/Geert Vanden Wijngaert)
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But the State Department issued a travel alert warning U.S. citizens in Europe to avoid crowded places, to be vigilant when in public or using mass transit and to exercise extra caution during religious holidays and at large festivals and events.

"Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants, and transportation," it said in a statement.

The Brussels bombings reverberated on the 2016 U.S. presidential campaign trail, with Democratic contender Hillary Clinton declaring that more needed to be done to confront the Islamic State militants who claimed responsibility for the attacks.

The Republican front-runner in the White House race, Donald Trump, called again for tighter border security and suggested U.S. intelligence services could use torture to head off future attacks.

SEE ALSO: Mormon missionaries from Utah among Belgian bombing survivors

Some of the country's busiest airports and other transportation facilities were placed on heightened security status, as illustrated by a greater law enforcement presence.

Large numbers of uniformed police officers and National Guard troops dressed in battle fatigues and carrying rifles patrolled New York's John F. Kennedy International Airport. Several U.S. carriers - Delta Air Lines Inc, United Continental Holdings Inc and American Airlines Group Inc - said they canceled or rerouted flights as a result of the Brussels attacks.

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At midafternoon, authorities at the Denver airport evacuated two levels on the west side of the main terminal after several packages that appeared suspicious were spotted near ticket counters, airport spokeswoman Stacey Stegman said.

Denver police, FBI and U.S. Transportation Security Administration officers converged on the airport, but the packages were ultimately deemed to pose no threat, and the terminal was fully reopened within two hours.

Several airlines were affected by the scare, including American Airlines, Aeroméxico, Air Canada, Lufthansa and British Airways, the airport said.

'WORLD MUST UNITE'

U.S. President Barack Obama ordered flags flown at half-staff in memory of the victims in the Belgium attacks.

The State Department said an undetermined number of U.S. citizens had been injured in Brussels but none were killed. Three Mormon missionaries and a U.S. Air Force member and his family were among those hurt.

The Obama administration also was expected to impose tighter security measures at U.S. airports following the Brussels Airport bombings, which occurred in a public hall outside of the security check area.

U.S. Representative William Keating of Massachusetts, senior Democrat on a House subcommittee on terrorism, said the suicide bombings illustrated the difficulty of protecting "soft targets" outside tightly controlled security cordons.

"The targets aren't going to be just getting on the plane itself, but the airport in general," he said in a phone interview.

Data curated by FindTheData

Obama addressed the attacks briefly in a speech in Havana on his historic visit to Cuba, vowing to support Belgium as it hunts for 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.

Candidates seeking their party's nomination for the Nov. 8 presidential election immediately weighed in, with Clinton, a former secretary of state, vowing to strengthen her drive to "defeat terrorism and radical jihadism."

World reacts to Brussels attacks

38 PHOTOS
World reacts to Brussels attacks -- tributes, memorials
See Gallery
Belgium bombings raise security alerts at US airports
TOPSHOT - A Belgian flag is display on the Trevi Fountain in Rome on March 22, 2016 in tribute to the victims of Brussels following triple bomb attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. Belgium launched a huge manhunt on March 22 after a series of bombings claimed by the Islamic State group ripped through Brussels airport and a metro train, killing around 35 people in the latest attack to bring carnage to the heart of Europe. / AFP / GABRIEL BOUYS (Photo credit should read GABRIEL BOUYS/AFP/Getty Images)
The Eiffel Tower is illuminated with the Belgium national colors black, yellow and red in honor of the victims of the today's attacks at the airport and the metro station in Brussels, in Paris, Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and its subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. (AP Photo/Thibault Camus)
The full moon rises behind One World Trade Center on March 22, 2016 in New York, it's mast displaying the colors of the Belgian flag in a show of support following the terrorist attacks in Brussels. / AFP / TREVOR COLLENS (Photo credit should read TREVOR COLLENS/AFP/Getty Images)
A picture taken on March 22, 2016 shows the Belgian flag projected on the European Union Commission building in tribute to the victims of Brussels following triple bomb attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. Belgium launched a huge manhunt on March 22 after a series of bombings claimed by the Islamic State group ripped through Brussels airport and a metro train, killing around 35 people in the latest attack to bring carnage to the heart of Europe. / AFP / PHILIPPE HUGUEN (Photo credit should read PHILIPPE HUGUEN/AFP/Getty Images)
A picture taken on March 22, 2016, shows the town council building in Belgrade illuminated in the colours of the Belgian flag in tribute to the victims of Brussels following attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. / AFP / Oksana Toskic / Serbia OUT (Photo credit should read OKSANA TOSKIC/AFP/Getty Images)
PARIS, FRANCE - MARCH 22: French people gather in front of the Hotel de Ville to show solidarity with Belgians during a commemoration, held with the attendance of French Minister of Foreign Affairs Jean Marc Ayrault, Belgium Ambassador to Paris Vincent Mertens de Wilmars and Mayor of Paris, Anne Hidalgo in Paris, France on March 22, 2016. At least 34 people were killed and more than 100 injured in multiple terrorist attacks at an airport and metro station in Brussels on today's morning. (Photo by Geoffroy Van der Hasselt/Anadolu Agency/Getty Images)
(COMBO) This combination of pictures created on March 22, 2016 shows colours of the Belgian flag being projected on to (from top L) the Brandenburg Gate in Berlin, the Eiffel Tower in Paris, the town council building in Belgrade, the Trevi Fountain in Rome, the Royal Palace at Dam Square in Amsterdam and Rome's Campidoglio in tribute to the victims of Brussels following the triple bomb attacks that killed about 35 people and left more than 200 people wounded. / AFP / AFP AND ANP / Serbia OUT - Netherlands OUT (Photo credit should read ODD ANDERSEN,LIONEL BONAVENTURE,OKSANA TOSKIC,GABRIEL BOUYS,EVERT ELZINGA,FILIPPO MONTEFORTE/AFP/Getty Images)
TOPSHOT - A couple embraces as the colors of the Belgian flag are projected on to the Brandenburg Gate in Berlin as the German capital shows its solidarity following the Brussels attacks on March 22, 2016. Security was tightened across Europe and transport links paralysed after a series of apparently coordinated explosions ripped through Brussels airport and a metro train, killing at least 34 people. / AFP / ODD ANDERSEN (Photo credit should read ODD ANDERSEN/AFP/Getty Images)
A picture taken on March 22, 2016 shows the Eiffel Tower in Paris illuminated in colours of the Belgian flag in tribute to the victims of terrorist attacks in Brussels. Around 35 people were killed and more than 200 wounded in a series of attacks in Brussels today claimed by the Islamic State group and described as a strike at the very heart of Europe. AFP PHOTO / LIONEL BONAVENTURE / AFP / LIONEL BONAVENTURE (Photo credit should read LIONEL BONAVENTURE/AFP/Getty Images)
PARIS, FRANCE - MARCH 22: (FRANCE OUT) Flags are hung on the Hotel de Ville to pay tribute to the victims of the terrorists attacks in Belgium on March 22, 2016 in Paris, France. At least 31 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 Nicolas Kovarik/IP3/Getty Images)
LONDON, ENGLAND - MARCH 22: The Belgian national flag is raised as a mark of respect at Downing Street on March 22, 2016 in London, England. 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 Ben Pruchnie/Getty Images)
TORONTO, ON- MARCH 22: Magali Labe from Belgium holds for a moment of silence at a vigil. The Toronto Sign outside City Hall sports black-yellow-red of Belgium in symapathy of those killed and injured in Brussels earlier in the day. at City Hall in Toronto. March 22, 2016. (Steve Russell/Toronto Star via Getty Images)
TORONTO, ON- MARCH 22: The Toronto Sign outside City Hall sports black-yellow-red of Belgium in symapathy of those killed and injured in Brussels earlier in the day. at City Hall in Toronto. March 22, 2016. (Steve Russell/Toronto Star via Getty Images)
ISTANBUL, TURKEY - MARCH 22: Belgian Consul to Istanbul Henri Marc Jose Vantieghem, US Consul to Istanbul Charles Frederick Hunter, Israel's Consul to Istanbul Shai Cohen, Mayors of Istanbul's Districts Beyoglu, Fatih, Esenler and Turkey's ruling party Justice and Development Party's (AK Party) Provincial members walks during a protest, condemning terrorism on March 22, 2016 in Istanbul, Turkey. Four people were killed and 36 injured, 7 of them seriously, in Saturdays suicide bomb blast on Istanbuls Istiklal Street. Also, at least 34 people were killed and more than 100 injured in multiple explosions at an airport and metro station in Brussels on today's morning. Metro stations were closed and flights to Zaventem airport diverted while the Eurostar rail service was also cancelled into and out of Brussels. (Photo by Islam Yakut/Anadolu Agency/Getty Images)
PARIS, FRANCE - MARCH 22: French people leave flowers to pay respect to Belgium after Brussels Terror attack, at the Belgian Embassy in Paris, France on March 22, 2016. (Photo by Geoffroy Van der Hasselt/Anadolu Agency/Getty Images)
PARIS, FRANCE - MARCH 22: French people leave French flag and flowers to pay respect to Belgium after Brussels Terror attack, at the Belgian Embassy in Paris, France on March 22, 2016. (Photo by Geoffroy Van der Hasselt/Anadolu Agency/Getty Images)
PARIS, FRANCE - MARCH 22: French people leave flowers to pay respect to Belgium after Brussels Terror attack, at the Belgian Embassy in Paris, France on March 22, 2016. (Photo by Geoffroy Van der Hasselt/Anadolu Agency/Getty Images)
UNITED STATES - MARCH 22 - Flowers are let below the Belgium flag as it flies at half staff at the Belgium Embassy in Washington D.C., Tuesday, March 22, 2016, after the terror attacks in Brussels, Belgium, earlier today. (Photo By Al Drago/CQ Roll Call)
UNITED STATES - MARCH 22 - Flowers are let below the Belgium flag as it flies at half staff at the Belgium Embassy in Washington D.C., Tuesday, March 22, 2016, after the terror attacks in Brussels, Belgium, earlier today. (Photo By Al Drago/CQ Roll Call)
Flowers wrapped in ribbons in the colours of the Belgium national flag, are placed outside the Belgium Embassy in Moscow, Russia on 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/ Pavel Golovkin)
Activists of Pakistani civil society chant slogans during a rally to condemn the Brussels attack, in Multan, Pakistan, Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lock down of the Belgian capital and heightened security across Europe. Placard at left reads, "we want peace in the world." (AP Photo/Asim Tanveer)
People hold hands as they gather at a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
A picture taken on March 22, 2016 in Paris shows a view of a smartphone screening an Instagram page with the ashtag '#JESUISBRUSSELS' (#IAMBRUSSELS) and two tributes images picturing the color of the Belgian flag, a drawing by French cartoonist Plantu and the famous Belgian comic character Tintin, in tribute to victims of triple bomb attacks in the Belgian capital that killed about 35 people. Airlines cancelled hundreds of flights and European railways froze links with Brussels on March 22, 2016 after rush-hour bomb attacks killed around 35 people in the city's airport and a metro train, sparking a broad security response. Within hours of Brussels attacks tens of thousands of people were sharing images on social media of Herge's cub reporter Tintin, the country's most famous creation, in tears. / AFP / JOEL SAGET / RESTRICTED TO EDITORIAL USE - MANDATORY MENTION OF THE ARTIST UPON PUBLICATION - TO ILLUSTRATE THE EVENT AS SPECIFIED IN THE CAPTION (Photo credit should read JOEL SAGET/AFP/Getty Images)
A picture taken on March 22, 2016 in Paris shows a view of a smartphone screening an Instagram page with the ashtag '#JESUISBRUSSELS' (#IAMBRUSSELS) and various tributes images picturing the Belgian flag, a drawing by French cartoonist Plantu or the famous Belgian comic character Tintin in tribute to victims of triple bomb attacks in the Belgian capital that killed about 35 people. Airlines cancelled hundreds of flights and European railways froze links with Brussels on March 22, 2016 after rush-hour bomb attacks killed around 35 people in the city's airport and a metro train, sparking a broad security response. Within hours of Brussels attacks tens of thousands of people were sharing images on social media of Herge's cub reporter Tintin, the country's most famous creation, in tears. / AFP / JOEL SAGET / RESTRICTED TO EDITORIAL USE - MANDATORY MENTION OF THE ARTIST UPON PUBLICATION - TO ILLUSTRATE THE EVENT AS SPECIFIED IN THE CAPTION (Photo credit should read JOEL SAGET/AFP/Getty Images)
Tributes to the victims of the terror attacks in Brussels are seen inside the Belgian Embassy in central London on March 22, 2016. Security was tightened across Europe and transport links paralysed after a series of apparently coordinated explosions ripped through Brussels airport and a metro train, killing at least 35 people. / AFP / JUSTIN TALLIS (Photo credit should read JUSTIN TALLIS/AFP/Getty Images)
People hold hands as they gather at a makeshift memorial at Place de la Bourse (Beursplein) following attacks in Brussels on March 22, 2016. Airlines cancelled hundreds of flights and European railways froze links with Brussels after a series of bomb blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response. AFP PHOTO / KENZO TRIBOUILLARD / AFP / KENZO TRIBOUILLARD (Photo credit should read KENZO TRIBOUILLARD/AFP/Getty Images)
MOSCOW, RUSSIA.MARCH 22, 2016. People bring flowers to the Belgian Embassy in Moscow to pay tribute to the Brussels terror attacks victims. On March 22, 2016, two explosions rocked Zaventem Airport and one blast struck a metro station. Mikhail Japaridze/TASS (Photo by Mikhail Japaridze\TASS via Getty Images)
A boy holds a placard expressing sympathy for the victims of the terror attacks in Brussels during a protest at a makeshift camp at the Greek-Macedonian border near the village of Idomeni on March 22, 2016. Greece will not be able to start sending refugees back to Turkey from March 20, 2016, the government said, as the country struggles to implement a key deal aimed at easing Europe's migrant crisis. The numbers are daunting: officials said as of Saturday there were 47,500 migrants in Greece, including 8,200 on the islands and 10,500 massed at the Idomeni camp on the Macedonian border. / AFP / ANDREJ ISAKOVIC (Photo credit should read ANDREJ ISAKOVIC/AFP/Getty Images)
After attacks in Brussels, a candle wrapped in a ribbon in the colors of the Belgium national flag and flowers are placed on a table inside the Belgium Embassy in Berlin, Germany, Tuesday, March 22, 2016. (AP Photo/Markus Schreiber)
Flowers are placed outside the embassy of Belgium, in Moscow, Russia, 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/Pavel Golovkin)
After deadly attacks on the Brussels airport and its subway system, a boy places a candle in front of the Belgian Embassy in Berlin, Germany, Tuesday, March 22, 2016. (AP Photo/Markus Schreiber)
MOSCOW, RUSSIA.MARCH 22, 2016. People bring flowers to the Belgian Embassy in Moscow to pay tribute to the Brussels terror attacks victims. On March 22, 2016, two explosions rocked Zaventem Airport and one blast struck a metro station. Mikhail Japaridze/TASS (Photo by Mikhail Japaridze\TASS via Getty Images)
MOSCOW, RUSSIA.MARCH 22, 2016. People bring flowers to the Belgian Embassy in Moscow to pay tribute to the Brussels terror attacks victims. On March 22, 2016, two explosions rocked Zaventem Airport and one blast struck a metro station. Alexander Shcherbak/TASS (Photo by Alexander Shcherbak\TASS via Getty Images)
A man lights a candle where flowers have being laid outside the Belgian embassy in Berlin on March 22, 2016. Security was tightened across Europe and transport links paralysed after a series of apparently coordinated explosions ripped through Brussels airport and a metro train, killing at least 34 people. / AFP / ODD ANDERSEN (Photo credit should read ODD ANDERSEN/AFP/Getty Images)
A wreath is layed on a table along flowers and a candle inside the Belgium embassy in Berlin on March 22, 2016 as the national and EU flag is reflected in the window. People left tributes to the victims outside the embassy after a series of apparently coordinated blasts ripped through Brussels airport and a train. / AFP / ODD ANDERSEN (Photo credit should read ODD ANDERSEN/AFP/Getty Images)
A man looks at flowers and a sign reading (Defy terror, protect freedom) outside the Belgium embassy in Berlin on March 22, 2016. People left tributes to the victims outside the embassy after a series of apparently coordinated blasts ripped through Brussels airport and a train. / AFP / ODD ANDERSEN (Photo credit should read ODD ANDERSEN/AFP/Getty Images)
MOSCOW, RUSSIA.MARCH 22, 2016. People bring flowers to the Belgian Embassy in Moscow to pay tribute to the Brussels terror attacks victims. On March 22, 2016, two explosions rocked Zaventem Airport and one blast struck a metro station. Alexander Shcherbak/TASS (Photo by Alexander Shcherbak\TASS via Getty Images)
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Trump, a billionaire businessman, 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."

His Republican rival, U.S. Senator Ted Cruz of Texas, strengthened his call for Obama to clip the flow of refugees from "countries with significant al Qaida or ISIS presence," and called for heightened police scrutiny of neighborhoods with large Muslim populations.

The attack raised worries among some U.S. Muslims that they could face more hostility, although mainstream Muslims have repeatedly denounced violence.

"The media hype and political manipulation heightens our concerns," said Sheikh Shaker Elsayed, imam of the Dar Al-Hijrah mosque in Virginia.

Some travelers expressed concern that new security measures at airports, which had already imposed extensive restrictions since the Sept. 11, 2001, attacks on New York and Washington, would increase inconvenience without improving safety.

"It already takes all day," said Hans Vermulst, 66, who was at New York's Kennedy airport trying to get home to the Netherlands after his connecting flight to Brussels was canceled. "We have to take it as it comes, but I'm not happy with it."

Data curated by FindTheData

(Additional reporting by Eric M. Johnson in Seattle; Idrees Ali, Julia Edwards, Mark Hosenball, Ian Simpson, Alana Wise and Susan Heavey in Washington and Suzannah Gonzales in Chicago; Writing by Scott Malone; Editing by Grant McCool and Peter Cooney)







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