Most people remember exactly where they were when the Twin Towers collapsed in a pile of rubble and fire in Lower Manhattan on Sept. 11, 2001. For many Americans, that morning's events acted as a signpost of sorts -- a defining moment for an older generation that last experienced this kind of collective mourning after President John F. Kennedy's assassination in 1963, and for millennials, for whom this was the first major news event of their lives.
In the 15 years since the attacks, anxieties about radicalism and violence have increased in the U.S. These fears have made terrorism a top-of-mind security concern for Americans, and led to the creation of the Department of Homeland Security and the Transportation Security Administration.
Since then, smaller-scale terrorist attacks -- and reports of foiled plots -- have kept Americans on edge. The so-called Beltway snipers had the Washington, D.C., region on high alert in 2002, and recent mass shootings in Southern California and Orlando have also kept terrorism in the news.
RELATED: Look back at the harrowing events of 9/11
15 most iconic images from September 11, 2001 and aftermath
15 most iconic images from September 11, 2001 and aftermath
Content in this photo gallery may be difficult for some to see -- viewer discretion is advised.
This September 11, 2001 file photo shows US President George W. Bush interrupted by his Chief of Staff Andrew Card(L) shortly after news of the New York City airplane crashes was available in Sarasota, Florida.
(Photo by PAUL J. RICHARDS/AFP/Getty Images)
Hijacked United Airlines Flight 175 (L) flies toward the World Trade Center twin towers shortly before slamming into the south tower as the north tower burns following an earlier attack by a hijacked airliner in New York City September 11, 2001. The stunning aerial assaults on the huge commercial complex where more than 40,000 people worked on an ordinary day were part of a coordinated attack aimed at the nation's financial heart. They destroyed one of America's most dramatic symbols of power and financial strength and left New York reeling. (REUTERS/Sean Adair)
The second tower of the World Trade Center explodes into flames after being hit by a airplane, New York September 11, 2001 with the Brooklyn bridge in the foreground. Both towers of the complex collapsed after being hit by hijacked planes.
(REUTERS/Sara K. Schwittek)
Photo shows the point of impact where a plane crashed into the North tower of the World Trade Center in New York City early September 11, 2001. Three hijacked planes crashed into major U.S. landmarks on Tuesday, destroying both of New York's mighty twin towers and plunging the Pentagon in Washington into flames, in an unprecedented assault on key symbols of U.S. military and financial power.
(Jeff Christensen / Reuters)
This 11 September 2001 file photo shows Marcy Borders covered in dust as she takes refuge in an office building after one of the World Trade Center towers collapsed in New York. Borders was caught outside on the street as the cloud of smoke and dust enveloped the area.
(Photo credit STAN HONDA/AFP/Getty Images)
A true-color image taken by the Enhanced Thematic Mapper Plus (ETM+) aboard the Landsat 7 satellite on September 12, 2001 shows New York City and the smoldering World Trade Center following the September 11, 2001 attacks in this handout photo courtesy of NASA. The image was captured at roughly 11:30 a.m. Eastern Daylight Savings Time.
A person falls to their death from the World Trade Center after two planes hit the Twin Towers September 11, 2001 in New York City.
(Photo by Jose Jimenez/Primera Hora/Getty Images)
The south tower of the World Trade Center collapses September 11, 2001 in New York City.
(Thomas Nilsson/ Getty Images)
This 11 September 2001 file photo shows pedestrians running from the scene as one of the World Trade Center towers collapses in New York City following a terrorist plane crash on the twin towers.
(DOUG KANTER/AFP/Getty Images)
Rescue operations at Ground Zero; Firefighters finding victims and searching for survivors at the wreckage of the World Trade Center Towers following Tuesday's Terrorist attack in New York, United states on September 14, 2001.
(Photo by Graham MORRISON/Gamma-Rapho via Getty Images)
Rescue workers carry fatally injured New York City Fire Depatment Chaplain, Father Mychal Judge, from one of the World Trade Center towers in New York City, early September 11, 2001. Both towers were hit by planes crashing into the buildings and collapsed a short time later.
The damaged area of the Pentagon building, where a hijacked commercial jetliner slammed into it September 11, 2001, is seen in this file photo with the U.S. Capitol Building in the background, at sunrise on September 16, 2001.
Firefighters raise a U.S. flag at the site of the World Trade Center after two hijacked commercial airliners were flown into the buildings September 11, 2001 in New York.
(Photo by 2001 The Record (Bergen Co. NJ)/Getty Images)
A New York City fireman calls for more rescue workers to make their way into the rubble of the World Trade Center September 15, 2001.
(REUTERS/Handout/U.S. Navy Photo by Journalist 1st Class Preston Keres)
Members of the New York Fire and Police Departments salute as a truck carrying the last steel column of the World Trade Center moves up West Street from inside of the World Trade Center site May 30, 2002 as the recovery effort at Ground Zero officially ends in New York.
(Photo by Spencer Platt/Getty Images)
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In 2015, there were 38 terrorist attacks or threats in the United States, according to data from the Global Terrorism Database. Although an unsettling tally, it doesn't come close to the number of attacks in the 1970s and 1980s. During that 20-year span, there was an average of about 99 terrorism attacks or threats per year, largely due to nationalist or anti-war groups.
Incidents of terrorism have fallen significantly since that peak in 1970. But the 38 terrorist attacks or threats in 2015 mark the highest count since 2001, when there were 40. As the InsideGov visualization below shows, the number of attacks and threats has been increasing fairly steadily again since 2011.
While death by terrorism doesn't register as a particularly common way to die in the U.S., the same can't be said for other countries. Take Iraq, for example, where 3,350 people have died so far in ISIL attacks in 2016, according to IntelCenter. That total would rank ISIL as among the top 15 causes of death in Iraq, using World Health Organization data from 2014 as a reference for leading causes of death in Iraq.
A recent InsideGov story looked at data specific to ISIL terrorist attacks in 2016. It pointed out that, in the U.S., you are about nine times more likely to die as a result of falling out of bed than in an ISIL terrorist attack.