In this Sept. 11, 2001 file photo, United Airlines Flight 175 collides into the south tower of the World Trade Center in New York as smoke billows from the north tower. | AP

FILE - In this Tuesday, Sept. 11, 2001 file photo, United Airlines Flight 175 approaches the south tower of the World Trade Center in New York moments before collision, seen from the Brooklyn borough of New York. (AP Photo/ William Kratzke)

FILE - In this Tuesday, Sept. 11, 2001 file photo, people run from a cloud of debris from the collapse of a World Trade Center tower in New York. (AP Photo/Suzanne Plunkett)

FILE - In this Tuesday, Sept. 11, 2001 file photo, people in front of New York's St. Patrick's Cathedral react as they look down Fifth Avenue towards the World Trade Center after two airliners crashed into the twin 110-story buildings. (AP Photo/Marty Lederhandler)

FILE - In this Tuesday, Sept. 11, 2001 file photo, two women hold each other as they watch the World Trade Center burn following a terrorist attack on the twin skyscrapers in New York. (AP Photo/Ernesto Mora)

FILE - In this Sept. 11, 2001 file photo, United Airlines Flight 175 approaches the south tower of the World Trade Center in New York shortly before collision as smoke billows from the north tower. (AP Photo/Carmen Taylor)

FILE - In this Tuesday, Sept. 11, 2001 file photo, the south tower starts to collapse as smoke billows from both buildings of the World Trade Center in New York. (AP Photo/Richard Drew)

FILE - EDS NOTE GRAPHIC CONTENT - In this Tuesday, Sept. 11, 2001 file photo, a person falls from the north tower of the World Trade Center in New York. (AP Photo/Richard Drew)

FILE - In this Tuesday, Sept. 11, 2001 file photo, Chief of Staff Andy Card whispers into the ear of President George W. Bush to give him word of the plane crashes at the World Trade Center, during a visit to the Emma E. Booker Elementary School in Sarasota, Fla. (AP Photo/Doug Mills)

FILE - In this Tuesday, Sept. 11, 2001 file photo, with the skeleton of the World Trade Center twin towers in the background, New York City firefighters work amid debris on Cortlandt St. after the terrorist attacks. (AP Photo/Mark Lennihan)

FILE - In this Tuesday, Sept. 11, 2001 file photo, a priest prays over a wounded man outside the west entrance of the Pentagon as emergency workers from all services help the wounded after a terrorist attack on the Department of Defense building in Washington. (AP Photo/Navy Times, Mark Faram)

FILE - In this Tuesday, Sept. 11, 2001 file photo, rubble and ash fill lower Manhattan streets after two hijacked airliners were crashed into the towers of the World Trade Center in New York, collapsing them. (AP Photo/Boudicon One)

FILE - In this Tuesday, Sept. 11, 2001 file photo, people flee lower Manhattan across the Brooklyn Bridge in New York after a terrorist attack on the World Trade Center. (AP Photo/Daniel Shanken)

FILE - In this Tuesday, Sept. 11, 2001 file photo, emergency workers look at the crater created when United Airlines Flight 93 crashed near Shanksville, Pa. (AP Photo/Keith Srakocic)

FILE - In this Tuesday, Sept. 11, 2001 file photo, Julie McDermott, center, is helped by others as they make their way through the debris near the World Trade Center in New York. (AP Photo/Gulnara Samoilova)

FILE - This Tuesday, Sept. 11, 2001 file photo shows the impact site of American Airlines Flight 11 in the north tower of the World Trade Center in New York. A person stands at the bottom center of the tear in the building. (AP Photo/Amy Sancetta)

FILE - In this Tuesday, Sept. 11, 2001 file photo, pedestrians flee the dust-filled area surrounding the World Trade Center following a terrorist attack on the New York landmark. (AP Photo/Amy Sancetta)

FILE - In this Tuesday, Sept. 11, 2001 file photo, the south tower starts to collapse as smoke billows from both buildings of the World Trade Center in New York. (AP Photo/Jim Collins)