Smoke, flames and debris erupts from one of the World Trade Center towers as a plane strikes it Tuesday, September 11, 2001. (AP Photo/ CHAO SOI CHEONG)
Smoke pours off one of the towers of the World Trade Center as flames explode from the second one as it is struck by a plane Tuesday, Sept. 11, 2001. (AP Photo/ CHAO SOI CHEONG)
A plane approaches New York's World Trade Center moments before it struck the tower at left, as seen from downtown Brooklyn, Tuesday, Sept. 11, 2001. (AP Photo/ WILLIAM KRATZKE)
A jet crashes into the World Trade Center in New York, Tuesday, Sept. 11, 2001. (AP Photo/ MOSHE BURSURKER)
Smoke billows from one of the towers of the World Trade Center and flames and debris explode from the second tower, Tuesday, Sept. 11, 2001. (AP Photo/ CHAO SOI CHEONG)
A fireball erupts from one of the World Trade Center towers as it is struck by the second of two airplanes in New York, Tuesday, Sept. 11, 2001. (AP Photo/ TODD HOLLIS)
Firefighters work beneath the destroyed mullions, the vertical struts which once faced the soaring outer walls of the World Trade Center towers. (AP Photo/ MARK LENNIHAN)
Two men hug in downtown Manhattan in response to the collapse of the World Trade Center's twin towers on September 11, 2001 in New York City. (AP Photo/ Louis Lanzano)
Smoke pours off one of the towers of the World Trade Center as flames explode from the second one as it is struck by a plane Tuesday, Sept. 11, 2001. (Photo/Louis Lanzano)
People flee the scene near New York's World Trade Center after terrorists crashed two planes into the towers on Tuesday, September 11, 2001. (AP Photo/ Diane Bondareff)
The north tower of the World Trade Center collapses on September 11, 2001 in New York City. (Photo/Diane Bondareff)
People flee the scene near New York's World Trade Center after terrorists crashed two planes into the towers on Tuesday, September 11, 2001. (AP Photo/ Diane Bondareff)
The north tower of the World Trade Center collapses on September 11, 2001 in New York City. (Photo/Diane Bondareff)
The north tower of the World Trade Center in flames on September 11, 2001 in New York City. (Photo/Diane Bondareff)
Ground zero on the evening of Sept. 11, 2001 after the September 11 terrorist attacks on the World Trade Center in New York City. (Photo/Mark Lennihan)
The New York City skyline on the evening of September 17, 2001 after the September 11 terrorist attacks on the World Trade Center in New York City. (AP Photo/ Mark Lennihan)
Emergency workers at ground zero on Sept. 11, 2001 after the terrorist attacks on the World Trade Center in New York City. (AP Photo/ Mark Lennihan)
A street near ground zero on the evening of September 11, 2001 after the September 11 terrorist attacks on the World Trade Center in New York City. (AP Photo/ Mark Lennihan)
A destroyed subway station near ground zero on the evening of Sept. 12, 2001 after the September 11 terrorist attacks on the World Trade Center in New York City. (AP Photo/ Mark Lennihan)
Debris fall from one of the burning twin towers of the World Trade Center after a hijacked plane crashed into the tower on September 11, 2001 in New York City. (AP Photo/ Richard Drew)
The north tower of the World Trade Center's twin towers burns after a hijacked plane crashed into it on September 11, 2001 in New York City. (AP Photo/ Louis Lanzano)
A man cries on September 11, 2001 after witnessing the collapse of the north tower of the World Trade Center in New York City. (AP Photo/ Shawn Baldwin)
The north tower of the World Trade Center collapses after a hijacked plane crashed into it on September 11, 2001 in New York City. (AP Photo/ Richard Drew)
Smoke rises from the burning twin towers of the World Trade Center after hijacked planes crashed into the towers on September 11, 2001 in New York City. (AP Photo/ Richard Drew)
A man is covered in debris after the collapse of the twin towers of the World Trade Center on September 11, 2001 in New York City. (AP Photo/ Richard Drew)
Debris fall from one of the burning twin towers of the World Trade Center after a hijacked plane crashed into the tower on September 11, 2001 in New York City. (AP Photo/ Richard Drew)
Debris fall from one of the burning twin towers of the World Trade Center after a hijacked plane crashed into the tower on September 11, 2001 in New York City. (AP Photo/ Richard Drew)