Seen in May 2014, the new One World Trade Center rises above New York City, just steps from the location of the former World Trade Center buildings. Those iconic landmarks were destroyed by terrorists on September 11, 2001, when nearly 3,000 people in New York, Virginia, and Pennsylvania lost their lives.
National Geographic photo editors have chosen 27 iconic images that tell stories from one of the country's darkest days.
Driven to the brink by fires and scorching temperatures during the 9/11 attacks, people near the top of the World Trade Center's north tower hang from windows as high as 1,300 feet (400 meters) above the streets of New York City.
Such images were extremely controversial in the days after the attacks but have since become more accepted—if no less disturbing, Chanin said.
"I think one of the things that's happened is that we recognize one way to represent the depravity at the core of the motivation for the attacks is by showing the people who were caught above the impact and the choices that they were confronted with at the ends of their lives. I think that's become an essential part of the story."
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.
Kehoe's Ladder 11 firehouse lost six men that day, but he survived to face a life forever changed not only by 9/11 but by the iconic image in which he unwittingly appeared.
"In some ways Mike Kehoe came to symbolize the firefighters," Chanin said.
Photograph courtesy John Labriola
Attack on the Pentagon
A video still shows American Airlines Flight 77 slamming into the western side of the Pentagon in Arlington, Virginia, at 9:37 a.m. ET on 9/11, claiming the lives of 59 persons on board.
In addition, 125 military and civilian employees at work inside the Pentagon were killed by the crash.
Still from video by CNN via Getty Images
Before the Collapse
People evacuate New York City's Financial District on 9/11 as both World Trade Center towers burn.
Within minutes, these scenes of orderly retreat would be replaced with images of widespread destruction and chaos, as the twin towers collapsed and choked city streets with dust and debris.
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.
(See a picture of the same scene after the twin towers collapsed.)
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.
The decision to publish or censor such images was polarizing in 2001.
"In the museum we are going to be showing images of people falling from the buildings, but they are going to be in a special alcove where people will have some type of advance notice of the content," said Chanin, of the national 9/11 memorial.
"We recognize that these images are not for everybody. But we also recognize that the conditions in extremis that these people found themselves in are a critical part of the story. And we can show that without identifying who those people are."
Photograph by Richard Drew, AP
The World Trade Center's south tower collapsed at 9:59 a.m. ET on 9/11, bringing the disaster that had been unfolding far above the street crashing down to engulf those below.
"I see a lot of images like these, and there is something about that frozen moment that captures the essence of what happened," Chanin said.
"While video incorporates the shock and the response and the aftermath, these particular photos get just that very moment. There is nothing really like them."
Photograph by Thomas Nilsson, Getty Images
Running for Their Lives
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.
The international nature of New York City meant that 9/11 had a truly global reach—citizens of some 115 nations were killed that morning.
Photograph by Suzanne Plunkett, AP
Firefighters battle a spreading blaze at the Pentagon in Arlington, Virginia, on 9/11.
Five minutes after the building was hit, the Federal Aviation Authority grounded all flights over or headed toward the continental United States. Less than ten minutes after the impact, evacuations began at the White House and the U.S. Capitol.
Photograph by Jim Varhegyi, U.S. Air Force via Getty Images
Escaping New York
New York City's workday world was turned on its head by the catastrophic events of 9/11.
This inbound view of the Brooklyn Bridge shows a route taken by many commuters earlier that morning turned into the site of a mass exodus as people walk out of a smoky and chaotic Manhattan.
Photograph by Daniel Shanken, AP
Assisting the Wounded
A wounded man outside the Pentagon's west entrance receives medical help from emergency workers on 9/11. A priest says prayers over him.
In addition to the Pentagon's 125 casualties, some 106 people were seriously injured by fires following the direct hit on the building, the epicenter of U.S. military strength.
Photograph by Mark Faram, Navy Times via AP
Enveloped in Ash
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.
The south tower burned for nearly an hour before it collapsed, claiming the lives of some 600 people inside the building and in the surrounding area.
The north tower burned for over an hour and a half before meeting a similar fate—-and killing another 1,400 people.
Photograph by Stan Honda, AFP/Getty Images
Street in Ruins
A lone person stands on a New York City street, seen after the twin towers' collapse on 9/11.
"Even after years, as I look at these photos they still seem really raw, and that quality of unreality that people expressed in the days just after the attack comes back to me as well," Chanin said.
"Sometimes, even after years, it's still hard to believe that this actually happened."
Photograph by Jason Florio, Corbis
The smoldering remains of the World Trade Center lie at the center of what would soon be dubbed ground zero. During the tenth anniversary of the attack, New York City Mayor Michael Bloomberg said the term should be phased out.
"We will never forget the devastation of the area that came to be known as ground zero. Never. But the time has come to call those 16 acres [6.5 hectares] what they are: The World Trade Center and the National September 11 Memorial and Museum," he said during a speech hailing rebuilding efforts.
Photograph by Alex Fuchs, AFP/Getty Images
Aiding the Injured
Todd Heaney and Frankie DiLeo of Engine 209 help a fellow firefighter who was injured at the World Trade Center during 9/11.
"The firefighter stories are so remarkable," Chanin said. "I can't tell you how many testimonies we've had from survivors of the buildings about their impressions of the firefighters as they were escaping down the stairways and the firefighters were going up those same stairs—as far as the 80th floor in one case.
"It's absolutely incredible how they conducted themselves in the midst of all this."
Photograph by Todd Maisel, NY Daily News via Getty Images
Skyline, Forever Changed
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
After the Crash
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 as part of the 9/11 plot, but interference from passengers forced the aircraft to crash in rural Shanksville.
Photograph by Tim Shaffer, Reuters
An injured rescue worker is pulled from the rubble of the World Trade Center on September 13, 2001.
In all, 343 firefighters and paramedics were killed on 9/11, as were 23 New York City police officers and 37 Port Authority police officers.
Photograph by Mario Tama, Getty Images
This picture of a piece of United Airlines Flight 93 lying in a Pennsylvania field on September 14 was introduced as evidence during the trial that linked Zacarias Moussaoui to al Qaeda and the 9/11 attacks.
Moussaoui was sentenced to life without parole in 2006.
Photograph from U.S. District Court via European Pressphoto Agency
The President's Men
U.S. President George W. Bush comforts New York City firefighter Lt. Lenard Phelan during a September 14 visit to ground zero.
Phelan's brother Kenneth, also a firefighter, was among hundreds of New York firefighters still missing in the wake of the 9/11 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.
Thousands of residents were forced to evacuate the area when the disaster left their homes uninhabitable.
Photograph by Todd Maisel, NY Daily News via Getty Images
Faces of the Fallen
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.
"I've heard from any number of survivors that they could tell by looking at firefighters' faces [the firefighters] knew how bad this was and that they might not get out," Chanin said. "They knew what they were facing, and the heroism and courage [they showed] is just astonishing."
Photograph by Mario Tama, Getty Images
Rising From the Rubble
On October 9, nearly a month after 9/11, the rubble of the World Trade Center's south tower still smoldered.
Today, years after 9/11, the memory of the attacks continues to burn even as a new version of the World Trade Center rises from the ashes at ground zero.
Architect Daniel Libeskind envisioned a World Trade Center memorial for what many consider hallowed ground. The eight-acre 9/11 Memorial Park includes two pools set in the footprints of the original Twin Towers.
The names of the 2,983 victims are inscribed on the parapets around the pools. The memorial opened on the tenth anniversary of the attacks.