A fiery blast rocks the south tower of the World Trade Center as the hijacked United Airlines Flight 175 from Boston crashes into the building Sept. 11, 2001, in New York City.
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Photo gallery: September 11, 2001

A fiery blast rocks the south tower of the World Trade Center as the hijacked United Airlines Flight 175 from Boston crashes into the building Sept. 11, 2001, in New York City.

 (Spencer Platt / Getty Images)

United Airlines Flight 175 approaches the south tower of the World Trade Center tower in New York after American Airlines Flight 11 slammed into the north tower about 20 minutes earlier.

 (KHBS/KHOG-TV)
United Airlines Flight 175 is seen moments before slamming into the south tower of the World Trade Center in New York.  (Sean Adair, Reuters)
A television image shows an airliner approaching the south tower, not visibe, of the World Trade Center. The aircraft crashed into the south tower just instants later. The north tower, in front, burns after being hit twenty mintues earlier by another hijacked airliner. Both towers collapsed to the ground after short time later. An aircraft also crashed into the Pentagon near Washington and another airliner crashed in Pennsylvania.  (Courtesy of NBC)

People hang from the windows of the north tower of the World Trade Center in New York City after a hijacked airliner crashed into the building.

 (Jose Jimenez / Primera Hora )

A person jumps from the north tower of the World Trade Center in New York as another clings to the outside, left, while smoke and fire billow from the building on Sept. 11, 2001.

 (Richard Drew / Associated Press)

The twin towers of New York City’s World Trade Center burn after two jetliners piloted by terrorists slammed into them on Sept. 11, 2001

 (Spencer Platt / Getty Images)

The south tower of the World Trade Center begins to collapse after a terrorist attack on the New York landmark on Sept. 11, 2001.

 (Amy Sancetta / Associated Press)

An explosion rocks one of the World Trade Center towers after a plane hit the building on Sept. 11, 2001.

 (Jose Jimenez / Primera Hora)

Firefighters head toward the north tower of the World Trade Center shortly before it collapsed after a plane piloted by terrorists struck the building on Sept. 11, 2001.

 (Jose Jimenez / Primera Hora)

People run away as the north tower of World Trade Center collapses after a hijacked airliner struck the building on Sept. 11, 2001.

 (Jose Jimenez / Primera Hora)

President George W. Bush‘s chief of staff, Andy Card, whispers into the president’s ear about the planes crashing into the World Trade Center as Bush was visiting Booker Elementary School in Sarasota, Fla., on Sept. 11, 2001.

 (Doug Mills / Associated Press)

A man covers his mouth to filter the air as he walks through debris after the collapse of one of New York’s World Trade Center towers on Sept. 11, 2001.

 (Stan Honda / AFP-Getty Image)

A firefighter screams in pain as he is rescued after being trapped under debris when one of the World Trade Center towers collapsed on Sept. 11, 2001.

 (Robert Mecea / Newsday)
Police, firemen and a civilian carry the body of FDNY Chaplain Mychal Judge after he was killed in the collapse of the south tower at the World Trade Center.  (Shannon Stapleton, Reuters)

A firefighter breaks down after the World Trade Center buildings collapsed after two hijacked airplanes slammed into the twin towers in a terrorist attack on Sept. 11, 2001.

 (Mario Tama / Getty Images)

People flee the scene near New York’s World Trade Center after terrorists crashed two planes into the twin towers on Sept. 11, 2001.

 (Diane Bondareff / Associated Press)

Horrified bystanders in front of New York’s St. Patrick’s Cathedral look down Fifth Avenue toward the World Trade Center towers after planes crashed into their upper floors.

 (Marty Lederhandler / Associated Press)

Two women hold each other as they watch the World Trade Center burn after a terrorist attack on the twin skyscrapers in New York on Sept. 11, 2001.

 (Ernesto Mora / Associated Press)
A man walks in the street near the World Trade Center towers in New York City. Both towers were hit by planes which crashed into the buildings, which collapsed shortly after.  (Shannon Stapleton, Reuters)
Firefighters comb the rubble of the World Trade Center after two hijacked commercial planes slammed into the twin towers, causing both 110-story landmarks to collapse in thunderous clouds of fire and smoke and killing a thousands of people. (Peter Morgan, Reuters)

People run from the collapse of the World Trade Center in New York on Sept. 11, 2001.

 (Suzanne Plunkett / Associated Press)
Pedestrians can be seen crossing the Brooklyn Bridge as they flee Manhattan after the collapse of the first World Trade Center tower.  (Doug Kanter, AFP/Getty Images)
A firefighter moves through piles of debris at the site of the World Trade Center. (Graham Morrison, AP)
The New York City skyline is changed forever after hijacked airliners crashed into the World Trade Center towers in 2001.  (Kathy Cacicedo, Newark Star-Ledger)
A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after an airplane crashed into the U.S. military Headquarters outside of Washington D.C.  (Larry Downing, Reuters)
A rescue helicopter flies over the damage to the Pentagon Building as fire fighters battle flames after American Airlines Flight 77 crashed into the U.S. military Headquarters outside of Washington.  (Larry Downing, Reuters)
Firefighters hose down the twisted and smoldering remnants of one side of the Pentagon after American Airlines Flight 77 crashed into the Pentagon’s west wall on the morning of Sept. 11.  (Steve Helber, AP)
Firefighters and emergency personnel investigate the scene of a fatal crash involving United Airlines Flight 93 near Shanksville, Pa. The San Francisco-bound Boeing 757, nosedived into the ground in an empty field in Somerset County, Pa., after meandering during its westward journey, leaving puzzled locals wondering why an airliner would be acting so wildly and reports of heart-breaking cell-phone calls home to bid farewell to loved ones.

The crash sparked speculation, reinforced by some comments from members of Congress, that the plane had been headed for Camp David, the presidential retreat in Maryland. Later, the list of potential targets was expanded to include the Pentagon, the White House or Air Force One.  (David Lloyd, Tribune-Democrat)
Firefighters and emergency personnel look through the debris of United Airlines Flight 93 after it crashed on the morning of Sept. 11, 2001 near Shanksville, Pa. 40 victims were killed in the crash after passengers reportedly stormed the cockpit to thwart terrorists’ plans to use that plane as a weapon like the others.  (David Maxwell, AFP/Getty)
A group of firefighters walk amid rubble near the base of the destroyed south tower of the World Trade Center in New York. In the worst terror attack on the U.S. mainland in modern history, two hijacked planes slammed into the twin towers of the World Trade Center in New York, a third crash into the Pentagon and a fourth crashed in Pennsylvania.  (Peter Morgan, Reuters)
The remains of the World Trade Center stand amid smoke and debris after the terrorist attack on the New York buildings.  (Alex Fuchs, AP)
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