See images from the moments and days that followed the 9/11 terrorist attacks on the World Trade Center in New York City.
Destroyed mullions are the only thing left standing behind a lone fireman, after a terrorist attack on the twin towers of lower Manhattan Tuesday, Sept. 11, 2001. (AP Photo/ MARK LENNIHAN)
Destroyed mullions are examined by emergency workers, after a terrorist attack on the twin towers of lower Manhattan Tuesday, Sept. 11, 2001. (AP Photo/ MARK LENNIHAN)
FIremen walk through a dust and debris covered street in lower Manhattan Tuesday, Sept. 11, 2001. (AP Photo/ RICHARD COHEN)
Rubble and ash fill lower Manhattan streets after two hijacked airliners were crashed into the towers of the World Trade Center in New York, Tuesday, Sept. 11, 2001. (AP Photo/ BOUDICON ONE)
Ladder 4 truck is surrounded by makeshift memorials as it returns to its midtown New York firehouse after a call Tuesday, Sept. 18, 2001. (AP Photo/ STUART RAMSON)
Work continued on the site of the World Trade Center as steam rises from the rubble in New York, Thursday, Oct. 11, 2001. (AP Photo/ MICHAEL CONROY)
A fallen piece of metal after hijacked planes crashed into the twin towers of the World Trade Center on September 11, 2001. (AP Photo/ Diane Bondareff)
In this September 21, 2001 photo, a dog has an American flag painted on its head as a memorial to the victims of the September 11 terrorist attacks. (AP Photo/ Diane Bondareff)
A woman looks at missing person posters of victims of the September 11 terrorist attacks on the World Trade Center in New York City on Sept. 14, 2001. (AP Photo/ Robert Spencer)
Construction workers continue to clear the rubble at the site of the World Trade Center, destroyed in the September 11, 2001 terrorist attacks, on September 15, 2001. (Photo/Wally Santana)
An area is marked searched at the ruins of the World Trade Center disaster in New York, Saturday Sept. 29, 2001. (AP Photo/ BEBETO MATTHEWS)
In this September 14, 2001 photograph, a man hold a candles and looks at missing person posters after the September 11 terrorist attacks on the World Trade Center in New York City. (AP Photo/ David Karp)
In this September 14, 2001 photograph, a man comforts a woman in front of missing person posters after the September 11 terrorist attacks. (AP Photo/ David Karp)
Construction workers continue to clear the rubble at the site of the World Trade Center, destroyed in the September 11, 2001 terrorist attacks, on September 15, 2001. (AP Photo/ Chad Rachman)
People light candles on Sept. 13, 2001 at a memorial for the victims of the September 11 terrorist attacks on the World Trade Center in New York City. (AP Photo/ Diane Bondareff)
Workers clean up rubble on September 15, 2001 at ground zero after the September 11, 2001 attacks in New York City (AP Photo/ Wally Santana)
Steel I-beams and rubble cover the tracks of the #1 and #9 subway lines in a tunnel under the World Trade Center. (AP Photo/ Anonymous)
A large portrait of Father Mychael Judge, the FDNY chaplain who died in the World Trade Center collapse, is part of the memorial to firemen at Ladder 24, Engine 1 (Photo/Charlie Krupa)
An ABC reporter in front of a car covered with posters of missing people from the September 11 terrorist attacks on the World Trade Center in NYC on Sept. 13, 2001. (AP Photo/ David Karp)
The casket for New York City firefighter Thomas Foley of West Nyack, N.Y., is carried from St. Anthony's Church in Nanuet, N.Y., to St. Anthony's cemetery. (AP Photo/ PETER CARR)
Two large cranes are positioned near the rubble of the World Trade Center in New York, on Wednesday, Sept. 26, 2001. (AP Photo/ QUYEN TRAN)
A man raises a flag at a prayer service on Sept. 15, 2001 after the September 11 terrorist attacks on the World Trade Center in New York City. (AP Photo/ Louis Lanzano)
Construction workers and firemen continue to clear the rubble at the site of the World Trade Center, destroyed in the September 11, 2001 terrorist attacks, on September 15, 2001. (AP Photo/ Wally Santana)
Construction workers and firemen continue to clear the rubble at the site of the World Trade Center, destroyed in the September 11, 2001 terrorist attacks, on September 14, 2001. (AP Photo/ Shawn Baldwin)
A lone firefighter moves through piles of debris at the site of the World Trade Center in New York, Tuesday, Sept. 11, 2001 (AP Photo/ GRAHAM MORRISON)