Billowing smoke choked the sky above New York City as Shannon Stapleton rushed towards the burning wreckage of the World Trade Center on Sept. 11, 2001.

With a camera in hand, the freelance photojournalist dodged pedestrians running from the scene and tore towards the base of the North Tower. That's where he captured images that are still etched into the public's collective memory of 9-11.

Resident New Yorkers spilled into city streets on Sept. 11 to witness the carnage caused by a pair of hijacked airliners that had barrelled into the Twin Towers. But millions of horrified onlookers living around the globe didn't have that same frame of reference.

For them, the 9-11 terror attacks have been illustrated by a stockpile of potent images and first-hand reports provided by journalists such as Stapleton.

"That day, I realized that I was covering the biggest assignment of my life in my own backyard," said Stapleton who covered 9-11 as a freelancer for Reuters, where he now works as a staff photographer.

Averting the glare of emergency officials, Stapleton stood at the base of the North Tower snapping photos of evacuees. Suddenly, he noticed a scene nearby that demanded to be captured.

Stapleton saw the lifeless body of a man in a white metal chair being carried out of the debris by two firemen, an NYPD lieutenant, an emergency worker and a civilian.

"I knew he was dead, I could just tell. And yet, it was beautiful how these men were really struggling to rescue him," said Stapleton.

Later, Stapleton would learn more about the man in the white chair. Father Mychal Judge was a friar who had rushed to the World Trade Center that day to administer last rites to victims. He was killed by a flying piece of debris that hit him when the South Tower collapsed.

On Sept. 14, Stapleton received a letter from some of Father Judge's family members.

"They wanted to thank me for taking the photo, for showing the world how much he loved the city he lived in," said Stapleton. "In that moment, I knew exactly why I was meant to be at the World Trade Center that day."

The presidential reaction

Ten years ago, what should have been a routine photo-op morphed into history in the making for veteran White House shutterbug Doug Mills.

The former Associated Press photographer was sitting in a Florida classroom on Sept. 11, 2001 waiting for then-U.S. president George W. Bush to read to a group of second-graders when something unusual happened.

Bush's Chief of Staff Andrew Card edged into the classroom and whispered something into the president's ear.

"You could tell he wasn't comfortable interrupting the president.'" recalled Mills. "His jaw was tight and his face was stern."

En route to the president, Card locked eyes with the group of journalists at the back of the classroom. He subtly held two fingers out. Later Mills would learn that Card's gesture meant two hijacked airliners had hit the World Trade Center in New York City.

"I knew it was a serious moment. I just didn't know it'd be that serious," said Mills. "I've been covering the White House since Ronald Reagan — the Chief of Staff never interrupts the president."

Kneeling on the classroom floor, Mills snapped a now-infamous photograph of Bush discovering that terrorists had targeted the United States. Though Bush continued reading to the students, Mills noted that he was visibly tense and appeared to have glassy eyes.

"Things like this never happen in front of the press. It's something we're never allowed to see," said Mills who currently works for The New York Times. "To have that raw emotion unfold right in front of us was remarkable."

Chaos under a microscope

On Sept. 11, 2011, Thomas Dallal's then-wife phoned him with a harrowing message and an explicit request.

"She told me she heard that a plane just hit the World Trade Center then said ‘whatever you do, don't go down there," Dallal recalled.

Minutes later, the former photojournalist did the exact opposite. With multiple cameras hanging from his neck, Dallal headed out of his home in New York's Chinatown and towards the Twin Towers.

It wasn't the first time Dallal bolted towards turmoil with a camera in tow. Before 9-11, he had snapped photos of refugees in Bangladesh, the first intifada and Israel's bombardment of south Lebanon.

Still, there was something different about the September 11 terrorist attacks.

"That day was jarring because it wasn't a situation I was prepared to walk into," he said. "I've covered mayhem in a lot of different places. This was mayhem in my front yard."

Briskly walking through the city's core, Dallal made a beeline for the West Side Highway where he could score an unobstructed view of the World Trade Center. Smoke billowed from the North Tower as he zoomed in on its top floors, capturing still images of people hanging out the windows in search of fresh air.

Dallal was focused on photographing on the two burning monoliths in the distance, until he heard two women sobbing heavily behind him. He turned to look at them and, for the first time all day, took stock of his rattled nerves.

"I realized that those women were watching the towers burn with just their eyes," he said. "I had a long lens. For me, it was like looking at chaos through a microscope. Mortifying."

In 2002, Dallal won an international accolade for his coverage of 9-11. His photograph of victims leaning out the North Tower windows netted a Pictures of the Year International (POYI) award.

Getting closer, much closer

Every time Mario Tama hears the cacophonous racket of a shopkeeper pulling a metal grate over a storefront, he's reminded of the World Trade Center on 9-11.

"To me, that's what it sounded like when the first tower's floors just started buckling onto each other," he said. "For months, I couldn't hear that sound without jumping."

Ten years ago, Tama had just moved to New York from Maryland to take a job with the stock photo agency Getty Images. The photographer, then 30, was still in bed at his Lower East Side apartment when his editor called to tell him that a plane had crashed into the WTC.

Unsure of what he was running towards, Tama threw on a small-brimmed hat, grabbed a pair of running shoes and slung a bag of camera equipment over his back.

"When I got outside to a spot where I could see the towers, I saw this huge gaping hole in the North Tower and I knew it wasn't an accident," he said.

It took about 15 minutes for Tama to jog to the World Trade Center, where a tsunami of dirt and debris would eventually greet him.

"The only instinct running through me was ‘You've got to get as close as you possibly can. Get closer. You're not close enough.'"

Battling the stinging scent of asbestos, Tama captured portraits of people around the base of the two towers. Later that day, he'd snap a thoughtful photograph of a New York firefighter kneeling in a pile of rubble with a gloved hand thrown over his face.

"Sometimes as a photographer you just know that you've taken a picture that'll have a lasting impact. For me, that was one of them," he said.

In the two weeks that followed Sept. 11, Tama returned to Ground Zero at least a dozen times to photograph the search for survivors. His favourite photo from that time shows a group of firefighters, standing in a row, removing debris from the site by hand.

"These guys were literally standing on top of the devastation," he said. "Just seeing their humanity shining through really got to me."

Shortly after snapping that photo, Tama wept for the first time since the 9-11 terror attacks.

"In that moment, I felt incredibly connected to the city," said Tama. "New York City can be a cold, impersonal place but the solidarity I witnessed that day was incredible."