** FILE **The south tower begins to collapse as smoke billows from both towers of the World Trade Center, in New York, in this Sept. 11, 2001, file photo. In one of the most horrifying attacks ever against the United States, terrorists crashed two airliners into the World Trade Center in a deadly series of blows that brought down the twin 110-story towers. (AP Photo/Jim Collins/FILE) ** zu unserem Korr **
Smoke billows from one of the towers of the World Trade Center as flames and debris explode from the second tower, Tuesday, Sept. 11, 2001. (AP Photo/Chao Soi Cheong)
Thick smoke billows into the sky from the area behind the Statue of Liberty, lower left, where the World Trade Center was, on Tuesday, Sept. 11, 2001. (AP Photo/Daniel Hulshizer)
Pedestrians in lower Manhattan watch smoke billow from New York's World Trade Center on Tuesday, Sept. 11, 2001. (AP Photo/Amy Sancetta)
Fire and smoke billows from the north tower of New York's World Trade Center on Tuesday, Sept. 11, 2001. (AP Photo/David Karp)
Two women hold each other as they watch the World Trade Center burn in New York Tuesday, Sept. 11, 2001. (AP Photo/Ernesto Mora)
A fireman screams in pain as he is rescued shortly after both towers of New York's World Trade Center collapsed following a terrorist attack, Tuesday, Sept. 11, 2001. (Robert Mecea/Newsday via AP)
People flee the scene near New York's World Trade Center Tuesday, Sept. 11, 2001. (AP Photo/Diane Bondareff)
A man coated with ash and debris from the collapse of the World Trade Center south tower collects himself near City Hall in lower Manhattan Sept. 11, 2001. (AP Photo/Amy Sancetta)
Deputy U.S. marshal Dominic Guadagnoli helps a women after she was injured in the terrorist attack on the World Trade Center in New York, Tuesday, Sept. 11, 2001. (AP Photo/Gulnara Samoilova)
Smoke billows through buildings in Manhattan as seen from Brooklyn after the collapse of New York's World Trade Center, Tuesday, Sept. 11, 2001. (AP Photo/Kathy Willens)
Pedestrians on Pierrepont Place in the Brooklyn borough of New York, watch as smoke billows from the remains of the World Trade Center in New York, Sept. 11, 2001. (AP Photo/Lawrence Jackson)
Deputy chief of the Army Reserve, Col. Malcolm Bruce Westcott, comforts Pentagon employee Racquel Kelley while giving her medical aid outside the Pentagon in Washington on Tuesday, Sept. 11, 2001. (AP Photo/Will Morris)
A 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)
People walk over New York's Brooklyn Bridge from Manhattan to Brooklyn following the collapse of both World Trade Center towers on Sept. 11, 2001. (AP Photo/Mark Lennihan)
People cover their faces as they escape the collapse of New York's World Trade Center on Sept. 11, 2001. (AP Photo/Suzanne Plunkett)
The remains of the World Trade Center stands amid the debris in New York, Tuesday, Sept. 11, 2001. (AP Photo/Alexandre Fuchs)
Pedestrians flee the area of New York's World Trade Center in lower Manhattan on Tuesday, Sept. 11, 2001. (AP Photo/Amy Sancetta)
People flee the falling South Tower of the World Trade Center on Tuesday, Sept. 11, 2001. (AP Photo/Amy Sancetta)
A person falls from the north tower of New York's World Trade Center as another clings to the outside, left, while smoke and fire billow from the building, Tuesday Sept. 11, 2001. (AP Photo/Richard Drew)
Firefighters work beneath the destroyed mullions, the vertical struts, of the World Trade Center in New York on Tuesday, Sept. 11, 2001. (AP Photo/Mark Lennihan)
Harry Shasho sweeps up before being evacuated from his vitamin store after the collapse of New York's World Trade Center on Tuesday, Sept. 11, 2001. (AP Photo/Suzanne Plunkett)
Flames and smoke pour from a building at the Pentagon in Washington on Tuesday, Sept. 11, 2001. (AP Photo/Will Morris)