Photos: A look back at scenes from the 9/11 attacks

One tower billows smoke while the other explodes
Smoke billows from World Trade Center’s north tower as the south tower explodes after it was struck by United Airlines Flight 175. For New Yorkers near the scene on Sept. 11, 2001, the sights and sounds of everyday life can still trigger painful memories and other psychological reactions to the attacks.
(Chao Soi Cheong / Associated Press)

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.

The Empire State Building in foreground and the smoking twin towers in the distance
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)
An explosion from a tower at left, and black smoke pouring from tower at right
The south tower, left, was struck at 9:03 a.m.
(Spencer Platt / Getty Images)
With the Statue of Liberty at left, the smoking Manhattan skyline after the attacks
Smoke rises over Manhattan after the terrorist attacks.
(Daniel Hulshizer / Associated Press)
People gather in a street watching smoke pouring from the twin towers.
A crowd in Lower Manhattan watches the twin towers before both collapsed.
(Amy Sancetta / Associated Press)
A woman is emotional as she looks skyward on a New York street
A Manhattan crowd stops to watch.
(Spencer Platt / Getty Images)
A person falls with a skyscraper in the background
A person falls from the north tower of the World Trade Center.
(Richard Drew / Associated Press)
A view up the tower as its top begins to fall, smoke pouring out
After burning for 56 minutes, the south tower begins to collapse at 9:59 a.m.
(Gulnara Samoilova / Associated Press)
People run away as smoke fills a street behind them
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)
People covered in dust and surrounded by debris walk down a street
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)
A woman is covered in dust
Marcy Borders is covered in dust as she takes refuge in an office building.
(Stan Honda / AFP/Getty Images)
A man covers his mouth as he walks amid dust and debris
Edward Fine covers his mouth and nose as he walks through Lower Manhattan.
(Stan Honda / AFP/Getty Images)
A man carries a fire extinguisher through a street filled with debris
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
A fireman screams in pain as he is rescued by fellow firefighters after the towers’ collapse.
(Robert Mecea / Associated Press)
People walk over Brooklyn Bridge, smoke billowing in the background
Pedestrians cross Brooklyn Bridge out of Manhattan.
(Mark Lennihan / Associated Press)
 An aide whispers in President Bush's ear as he sits in a classroom
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)
A helicopter flies over the smoking Pentagon building
Smoke pours from the Pentagon after it was attacked.
(Heesoon Yim / Associated Press)
Firefighters are dwarfed by debris at the World Trade Center site
Firefighters work at the site of the World Trade Center on Sept. 11, 2001.
(Graham Morrison / Associated Press)
A firefighter holds up 10 fingers, standing before smoldering debris
A New York City firefighter calls for 10 more rescue workers at the World Trade Center site.
(Preston Keres / Getty Images)
Rubble and smoke.
The rubble of the World Trade Center smolders.
(Alexandre Fuches / AFP/Getty Images)