The World Trade Center's south tower collapsed at 9:59 a.m. on 9/11, bringing the disaster that had been unfolding far above the street crashing down to engulf those below.
Photograph by Thomas Nilsson, Getty Images
At 9:03 a.m. eastern time, United Airlines Flight 175 crashed into the World Trade Center's south tower, as seen in a still from a video taken from the ground on September 11, 2001. By hijacking four planes, terrorists made a deadly attack.
Still from Video by Evan Fairbanks, Magnum Photos
Smoke and flames billow as United Airlines Flight 175 crashes into the World Trade Center's south tower on 9/11, killing everyone aboard and hundreds more inside the building.
Photograph by Spencer Platt, Getty Images
Driven to the brink by fires and scorching temperatures during the attacks, people near the top of the World Trade Center's north tower hang from windows as high as 1,300 feet (400 metres) above the streets of New York.
Photograph by Jose Jimenez, Primera Hora/Getty Images
While most able-bodied occupants of the north tower fled down stairwells to safety, firefighters such as Mike Kehoe (pictured) headed up to help the wounded.
Photograph by John Labriola
A video still shows American Airlines Flight 77 slamming into the western side of the Pentagon in Arlington, Virginia, at 9:37 a.m. on 9/11, claiming the lives of 59 persons on board and 125 on the ground.
Photograph by Still from video by CNN via Getty Images
People evacuate New York City's Financial District on 9/11 as both World Trade Center towers burn.
Photograph by Spencer Platt, Getty Images
The twin towers burn behind one of New York City's iconic landmarks, the Empire State Building, on 9/11.
Photograph by Marty Lederhandler, Ap
This famous photograph, known as "Falling Man," captures the plunge of an unknown victim of 9/11 from the north tower—one of many who jumped or fell to their deaths from the upper floors of the World Trade Center.
Photograph by Richard Drew, Ap
People run through the New York streets as the World Trade Center collapses behind them, blotting out blue skies and filling the air with enormous clouds of debris and ash.
Photograph by Suzanne Plunkett, Ap
Firefighters battle a spreading blaze at the Pentagon.
Photograph by Jim Varhegyi, U.S. Air Force via Getty Images
This inbound view of the Brooklyn Bridge shows a mass exodus as people walk out of a smoky and chaotic Manhattan.
Photograph by Daniel Shanken, Ap
A wounded man outside the Pentagon's west entrance receives medical help from emergency workers while a priest says prayers over him.
Photograph by Mark Faram, Navy Times, via Ap
Marcy Borders is enveloped in ash after she escaped the World Trade Center's south tower to take shelter in the lobby of a nearby office building. She passed away in 2015 after battling addiction and then cancer.
Photograph by <p> Stan Honda, AFP/Getty Images</p>
A lone person stands on a New York City street, seen after the Twin Towers' collapse on 9/11.
Photograph by Jason Florio, Corbis
The smoldering remains of the World Trade Center lie at the centre of what would soon be dubbed 'ground zero'.
Photograph by Alex Fuchs, AFP/Getty Images
Todd Heaney and Frankie DiLeo of Engine 209 help a fellow firefighter who was injured at the World Trade Center during 9/11.
Photograph by Todd Maisel, NY Daily News via Getty Images
The buildings of lower Manhattan are engulfed in clouds of smoke and debris as seen from Jersey City across the Hudson River shortly after the second tower collapsed on 9/11.
Photograph by Steffan Kaplan, New York Times, via Redux
Smoke rises behind investigators on September 12 as they comb the Pennsylvania field where United Airlines Flight 93 crashed. The plane was among four hijacked on 9/11 but crashed after interference from passengers.
Photograph by Tim Shaffer, Reuters
An injured rescue worker is pulled from the rubble of the World Trade Center on September 13, 2001.
Photograph by Mario Tama, Getty Images
This picture of a piece of United Airlines Flight 93 lying in a Pennsylvania field was introduced as evidence during the trial that linked Zacarias Moussaoui to the 9/11 attacks. He was sentenced to life without parole in 2006.
Photograph by U.S. District Court, via Epa
U.S. President George W. Bush comforts New York City firefighter Lenard Phelan during a September 14 visit to ground zero. Phelan's brother Kenneth, also a firefighter, was among hundreds of New York firefighters missing in the wake of the attacks.
Photograph from Fdny, via Ap
Seen on September 14, an apartment on Liberty Street in lower Manhattan is a burned-out shell following the collapse of the World Trade Center during the 9/11 attacks.
Photograph by <p> Todd Maisel, NY Daily News via Getty Images</p> <p> </p>
People on a New York City street scan pictures of the many police and fire personnel still missing in the wake of the attacks as of September 28, 2001.
Photograph by Mario Tama, Getty Images
On October 9, nearly a month after 9/11, the rubble of the World Trade Center's south tower still smoldered.
Photograph by <p> Michael Conroy, Associated Press</p>
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NEW YORK, NY - MAY 27: One World Trade Center stands on May 27, 2014 in New York City. With months before it was scheduled to open, the owners of One World Trade were cutting office rents due to a scarcity of renters at the iconic location built near the former location of the original World Trade Center buildings, which were destroyed in the September 11 terrorist attacks. With only 55% of current space leased, owners and developers of the building cut rents by 10% to £52 ($69) a square foot down from £57 ($75) a square foot. (Photo by Spencer Platt/Getty Images)
Photograph by Spencer Platt, Getty Images
The eight-acre 9/11 Memorial Park opened on the tenth anniversary of the attacks. It includes two pools set in the footprints of the original Twin Towers and the names of the 2,983 victims.
Photograph by Robert Wallis, Getty Images