Images that document enormity of 9/11 attacks

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.

Pedestrians in lower Manhattan watch smoke billow from New York's World Trade Center on Tuesday, Sept. 11, 2001.

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.

Two women hold each other as they watch the World Trade Center burn in New York Tuesday, Sept. 11, 2001.

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. 

People flee the scene near New York's World Trade Center Tuesday, Sept. 11, 2001. 

Pedestrians flee the area of New York's World Trade Center in lower Manhattan on Tuesday, Sept. 11, 2001. 

Fire and smoke billows from the north tower of New York's World Trade Center on Tuesday, Sept. 11, 2001. 

People flee the falling South Tower of the World Trade Center on Tuesday, Sept. 11, 2001.

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.

Flames and smoke pour from a building at the Pentagon in Washington on Tuesday, Sept. 11, 2001. 

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.

Firefighters work beneath the destroyed mullions, the vertical struts, of the World Trade Center in New York on Tuesday, Sept. 11, 2001.

Smoke billows through buildings in Manhattan as seen from Brooklyn after the collapse of New York's World Trade Center, Tuesday, Sept. 11, 2001. 

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.