Nearly 3,000 people died on the day of a series of coordinated attacks against the United States by terrorists who hijacked four passenger jets that crashed into the World Trade Center buildings in New York, the Pentagon and a field in rural Pennsylvania.<br><br>Smoke rises from the World Trade Center in the Manhattan borough of New York after the terror attack on Sept. 11, 2001.<br><br><strong>Warning: This slideshow contains images that some may find disturbing. Viewer discretion is advised.</strong>

American Airlines Flight 11 hit the North Tower at 8:46 a.m., crashing through floors 93 through 99. United Airlines Flight 175 hit the South Tower just 17 minutes later, crashing into floors 77 through 85.</br></br>A fiery blasts rocks the World Trade Center after being hit by two planes on Sept. 11, 2001, in New York City.

White House chief of staff Andrew Card whispers into the ear of President George W. Bush to give him word that a second plane crashed into the World Trade Center and that America was under attack, during a visit to the Emma E. Booker Elementary School in Sarasota, Fla., Sept. 11, 2001.

American Airlines Flight 77, a nonstop flight from Boston to Los Angeles, crashed into the Pentagon in Arlington, Va., at 9:37 a.m. with 59 passengers and crew and 5 hijackers.<br><br>Smoke rises from the Pentagon as viewed from executive offices of the Department of Veterans Affairs in Washington, D.C.

After burning for 56 minutes, the South Tower of the World Trade Center collapses, killing more than 800 people in and around the building.

People run from a cloud of dust and debris as one of the World Trade Center towers collapses on Sept. 11, 2001, in New York.

People watch from Jersey City, N.J. as the North Tower dissolves in a cloud of dust and debris about a half hour after the South Tower collapsed. More than 1,600 people in and around the building are killed. Only 102 minutes passed between the time of the first plane crash and the collapse of the second tower.

FBI personnel excavate the site where United Flight 93 crashed near Shanksville, Pa., Sept. 16, 2001.<br><br>United Flight 93 was scheduled as a nonstop flight from Newark to San Francisco, and is believed to have been hijacked at 9:28 a.m. According to the 9/11 Commission Report, passengers began to revolt at 9:57 a.m. before it ultimately crashed at 10:03 a.m., killing forty people and four hijackers.

Smoke billows from the Southwest E-ring of the Pentagon building after American Airlines Flight 77 crashed into the building and set off a huge explosion. The outer ring of the building collapsed at 10:15 a.m. The incident killed 64 on the plane, including 5 hijackers, and 125 people on the ground.

People cross over the Williamsburg Bridge into Brooklyn while smoke rises from the World Trade Center. At 11:02 a.m., New York City Mayor Rudy Giuliani ordered the evacuation of Lower Manhattan.

A New York firefighter works amid the rubble of the World Trade Center following the Sept. 11, 2001 terror attacks in New York. According to the NYFD, 343 firefighters were killed responding to the site of the attack.

Police officer Mike Brennan helps a distraught woman covered in ash and debris seek shelter following the collapse of the North Tower. In the years since the attack, thousands of first responders and survivors have died due to cancer or aerodigestive disorders, according to the CDC.

A bicyle sits covered with sediment and ash after the World Trade Center was hit by two planes, Sept. 11, 2001 in New York City.

Marcy Borders, 28, is covered in dust as she takes refuge in an office building after one of the World Trade Center towers collapsed. She was working on the 81st floor of the North Tower when Flight 11 crashed into the building. She died of stomach cancer in 2015 at the age of 42.

An unidentified New York City firefighter walks away from Ground Zero after the collapse of the Twin Towers, Sept. 11, 2001, in New York City. The World Trade Center's Twin Towers and the Pentagon were attacked by terrorists using commercial airliners.

Rescue crews engulfed in a fog of dust set about the task of finding survivors in the rubble of the World Trade Center, Sept. 11, 2001, in New York City, after a coordinated attack brought down the two towers.

Capt. Michael Dugan hangs an American flag from a light pole in front of what is left of the World Trade Center after it was destroyed in a terrorist attack, Sept. 11, 2001, in New York City.

The rubble of the World Trade Center smolders following a terrorist attack on Sept. 11, 2001, in New York.<br><br>According to a report released by the FDNY, Nearly 16,000 FDNY members were exposed to dust, particulates, noxious gases, chemicals and fibers while working for more than 10 months in the rescue and recovery effort.

FDNY firefighter, Dan Potter, pauses on a bench in Lower Manhattan after the collapse of the Twin Towers.

Firefighters raise a U.S. flag at the site of the World Trade Center after two hijacked commercial airliners were flown into the buildings, Sept. 11, 2001 in New York.

Handwritten notes in memory of the victims of the terrorist attack on the World Trade Center on Sept. 11, 2001, are seen in New York City.

A worker wipes his brow in the wreckage of the World Trade Center, Sept. 13, 2001, in New York City, two days after the twin towers were destroyed by two hijacked passenger jets.

Dennis Diaz, a member of Local 100 of the SEIU, looks over the Wall of Prayers, Sept. 13, 2001, at the entrance to Bellevue Hospital in New York City, for members of the union who are missing after the deadly terrorist attack on the World Trade Center.

A postcard pictured with the Twin Towers lies amid the rubble of the World Trade Center on Sept. 14, 2001.<br><br>According to Brown University's "Cost of War" project, the total death toll from wars in Afghanistan and Iraq may top 450,000.

The Statue of Liberty and One World Trade Center are seen as the Tribute in Light shines in downtown Manhattan to commemorate the 19th anniversary of the Sept. 11 attacks on the World Trade Center in New York, Sept. 11, 2020.