Twenty years after terrorists crashed hijacked planes into New York’s World Trade Center, the Pentagon and a field in Pennsylvania, the U.S. is still reckoning with the attack that toppled the twin towers, killed nearly 3,000 people, triggered a wave of increased security and launched a war on terrorism.

       Smoke pours from the twin towers over Manhattan on the morning of Sept. 11, 2001. After they collapsed, the Empire State Building, foreground, was the tallest building in New York.

       (Marty Lederhandler / Associated Press)

       The south tower, left, was struck at 9:03 a.m.

       (Spencer Platt / Getty Images)

       Smoke rises over Manhattan after the terrorist attacks.

       (Daniel Hulshizer / Associated Press)

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       A crowd in Lower Manhattan watches the twin towers before both collapsed.

       (Amy Sancetta / Associated Press)

       A woman looks up at the twin towers after the attack.

       (Spencer Platt / Getty Images)

       A person falls from the north tower of the World Trade Center.

       (Richard Drew / Associated Press)

       After burning for 56 minutes, the south tower begins to collapse at 9:59 a.m.

       (Gulnara Samoilova / Associated Press)

       The north tower, the first to be hit and the last to fall, collapsed at 10:28 a.m., 102 minutes after it was attacked.

       (Jose Jimenez / Primera Hora)

       Survivors of the World Trade Center attacks make their way through smoke, dust and debris on Fulton Street, about a block from the collapsed towers.

       (Gulnara Samoilova / Associated Press)

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       Marcy Borders is covered in dust as she takes refuge in an office building.

       (Stan Honda / AFP/Getty Images)

       Edward Fine covers his mouth and nose as he walks through Lower Manhattan.

       (Stan Honda / AFP/Getty Images)

       A man walks with a fire extinguisher, calling as he looks for survivors.

       (Doug Kanter / AFP/Getty Images)

       A fireman screams in pain as he is rescued by fellow firefighters after the towers’ collapse.

       (Robert Mecea / Associated Press)

       Pedestrians cross Brooklyn Bridge out of Manhattan.,

       (Mark Lennihan / Associated Press)

       White House Chief of Staff Andrew Card delivers news of the terrorist attacks to President Bush, who was visiting Emma E. Booker Elementary School in Sarasota, Fla.

       (Doug Mills / Associated Press)

       Smoke pours from the Pentagon after it was attacked.

       (Heesoon Yim / Associated Press)

       Firefighters work at the site of the World Trade Center on Sept. 11, 2001.

       (Graham Morrison / Associated Press)

       A New York City firefighter calls for 10 more rescue workers at the World Trade Center site.

       (Preston Keres / Getty Images)

       The rubble of the World Trade Center smolders.

       (Alexandre Fuches / AFP/Getty Images)