More buildings within the World Trade Center continue to burn and collapse, days after the initial terrorist attack that destroyed the Twin Towers. (Reuters Graphic)

Rescue workers are dwarfed by huge mountains of debris as they search for survivors for a third straight day, September 13, 2001, in the rubble from the collapse of the World Trade Center complex in New York City. Two hijacked commercial jetliners were deliberately crashed into the complex's twin towers on September 11. REUTERS/Jim Bourg

One of the towers of the World Trade Center collapses after being struck by a hijacked commercial airplane, in New York September 11, 2001. Several buildings in the trade center complex collapsed following a terrorist attack. REUTERS/Petra Beter

This series of photographs shows hijacked United Airlines Flight 175 as it approaches (upper L) and impacts the World Trade Center's south tower (L), bursting into flames and raining a hail of debris on lower Manhattan September 11, 2001. A gaping hole in the north tower (R) can be seen following a similar attack earlier in the day. (Sean Adair/Reuters)

Rescue and emergency workers search for survivors through the rubble and debris of the World Trade Center complex in New York City, September 13, 2001. Two hijacked commercial jetliners were deliberately crashed into the twin towers, bringing them crashing to the ground on September 11. REUTERS/Jim Bourg

Firemen, lower left, climb over the rubble of the World Trade Center as rescue operations continue in New York September 12, 2001. New York Mayor Rudolph Giuliani said that 4,763 people are counted as missing from the attack. (Shannon Stapleton/Reuters)

A mail truck burns next to a burned out police car after one of the twin towers of the World Trade Center collapsed following an aerial attack by hijacked airliners in New York September 11, 2001. Two hijacked commercial planes slammed into the twin towers on Tuesday, causing both 110-story landmarks to collapse in thunderous clouds of fire and smoke. REUTERS/Robby Berman
(Note: I think it's a fire engine...)

The World Trade Center south tower (L) is engulfed in flames after being struck by hijacked United Airlines Flight 175 as the north tower burns following an earlier attack by a hijacked airliner in New York City September 11, 2001. The stunning aerial assaults on the huge commercial complex where more than 40,000 people worked on an ordinary day were part of a coordinated attack aimed at the nation's financial heart. They destroyed one of America's most dramatic symbols of power and financial strength and left New York reeling. REUTERS/Sean Adair

Smoke from the remains of New York's World Trade Center shrouds lower Manhattan as the Statue of Liberty stands in this image taken across New York Harbor from Jersey City, New Jersey, September 12, 2001. Each of the twin towers were hit by hijacked airliners and collapsed in acts of terrorism directed at the United States September 11, 2001 (Ray Stubblebine/Reuters)

FILE--Osama bin Laden speaks to a selected group of reporters in the mountains of Helmand province in southern Afghanistan in this Dec. 24, 1998, file photo. U.S authorities investigating the Tuesday Sept. 11, 2001 devastating attacks on the World Trade Center and the Pentagon are focusing on multiple separate terrorist groups, some tied to bin Laden, law enforcement officials said Wednesday. (AP Photo/Rahimullah Yousafzai)