From 110 storeys, a distance of over 1,300ft, it was impossible at first to see what it was that was falling. One witness said it looked like confetti.

Perhaps it was debris: in a desperate attempt to escape as the World Trade Center towers burned around them, workers were hurling chairs or tables through the windows to reach fresh air before they were rescued. In those early minutes, a rescue operation seemed plausible.

Then two women on the ground, staring up at the gaping hole left in the North Tower by American Airlines Flight 11, clutched at each other and started screaming. It was people that were falling from the towers. Trapped above the point of impact, many witnesses concluded that they were jumping. “The Jumpers”, as they became known, were one of the most graphic and controversial elements of 9/11.

Thomas Dallal, a photojournalist at the time, was on the ground near the North Tower. He photographed the two women crying, put a long lens on his camera and turned back to the tower snapping away at the uppermost floors. One of his photographs, to be named “Impending Death”, has become an iconic image from that day. It shows around 50 figures leaning out of the broken windows of the North Tower shortly before it collapsed.

News organisations decided not to use footage of the people falling to their deaths. No one wanted to talk about the jumpers. Many still refuse to accept that they jumped, rather that they fell or were forced out by flames and explosions. However, as the years have passed, these images capture what, for many, are acts of heroism. Some fell holding hands. Others appeared to have made makeshift parachutes from clothing knotted together.

Richard Griffiths, the senior editorial director at CNN’s headquarters in Atlanta, was in the newsroom that day. “There was a robust discussion, and we did include those images that night in a special report – a four-second shot of someone jumping – but you didn’t see them hit the ground,” he says. “You got a clear sense of the awfulness of the attack. It’s that intimacy of seeing someone die; it is a very invasive thing to witness a death.

“Usually the experience we have of someone dying is through a parent, or a close relative; you are holding their hand and it is an intensely personal moment. There are thousands of people for whom that person jumping could have been their relative; and they will never know because so many of the bodies were not recovered.”

These were some of the most chilling images of the day. “The Falling Man”, taken by Richard Drew at the Associated Press, sparked a still-contested investigation into the identity of the man. He was pictured perfectly bisecting the towers, upside down, one knee bent, like an arrow in free-fall in a white jacket and orange T-shirt. The photograph became the subject of a documentary in 2006.

“Nobody is ever really going to know what happened, but I don’t think they were jumping,” says Dallal. “I think they were falling after being overwhelmed by smoke and heat. I think some were forced out by explosions inside. When you could see what was going on up there, it was calamitous.”

Dallal, 47, had been a photojournalist for 14 years covering war zones across the world. “When you cover a conflict as a journalist you go somewhere prepared psychologically. You are well aware that you are taking risks. But this happened in a part of town that was as familiar to me as my own front yard. It felt like I was watching people I ride the subway with every day going through a living hell. Did I understand that I was shooting something incredible? I wasn’t even thinking about it. I was on autopilot and I was frightened by what I was seeing.”

People jumped or fell from all four sides of both towers. USA Today estimated that around 200 people died in this way. The New York Times ran a more conservative estimate of 50.

It took 10 seconds for each person to fall, it was calculated, as they accelerated at 32ft per second achieving a speed of 150mph. Some who witnessed the jumpers see only desperation. Others see freedom: choosing how to die as a final act of defiance.

Dallal had been working at his home in downtown Manhattan when his then wife called to say a plane had hit the North Tower. She was at a photo-shoot at a loft on Mercer Street which runs due north from the World Trade Centre up through Soho.

“The first plane flew in low and fast over that loft,” says Dallal. “It shook the windows and the dogs started barking, then there was a huge explosion. I had heard the explosion where I lived in Chinatown. I knew it wasn’t fireworks – if you have ever been around real explosions, like bombs and mortar shells, not only is it loud, it has a 'woomph’ to it: you feel it.”

With several cameras slung around his neck, he walked the 15 minutes from his apartment to the World Trade Centre. He saw a smouldering piece of metal, part of a plane engine or the landing gear, he thought. There was the sound of breaking glass and debris in the air. “I got spooked and I thought, 'I don’t think I should be here,’” he says.

After “Impending Death” ran as a small image in the New York Times at the end of 2001, Dallal received a call from a woman who had lost both her sons in the North Tower. She wanted a copy of the picture enlarged, to see if she could identify her boys. Another woman who had lost her fiancé contacted Dallal with the same request.

“People were looking for closure,” he says. “I would have done anything in my power to help anyone who had lost a relative. But I didn’t want to compound their anguish. I felt responsible to be very careful with these people; they had already endured more than anyone would want to endure.”

Dallal invited the woman who had lost both sons to his house. “After a long conversation I asked her: 'Are you sure want to see the pictures in high resolution?’” She did.

Through Dallal’s images and from pictures taken from another angle by a Reuters photographer, the woman thought she was able to make a positive identification of her two sons together at the top of the tower.

“I have got to say that was one of the most moving, difficult experiences as a photo journalist,” says Dallal. He has since given up journalism, went to law school and is now an attorney for the UN agency UNRWA, based in Jerusalem. “I am still traumatised,” he says. “It’s such an unpleasant image I don’t like to look at it. It’s grisly. It’s a bad memory.”

In the 2003 Esquire magazine essay about Richard Drew’s “The Falling Man”, Tom Junod, the award-winning American journalist, wrote: “Some people who look at the picture see stoicism, willpower, a portrait of resignation; others see something else… There is something almost rebellious in the man’s posture, as though once faced with the inevitability of death, he decided to get on with it; as though he were a missile, a spear, bent on attaining his own end.”

Richard Drew, 64, who was one of four press reporters present when Robert F Kennedy was assassinated, says he “started to think about The Falling Man like The Unknown Soldier, representing all the people who met the same fate that day.”

If Drew hears the sound of a plane over the city it’s still “a trigger”, he says. “It was like watching a train wreck: people were staring up at the buildings, you turn away and yet you look back.”

But it has never been the photographer’s job to analyse or romanticise the images taken in war zones, or of catastrophes such as 9/11, says Dallal. “Ten years later, recalling that image and so many scenes that day, I see only innocent victims.”

What one man thought initially was debris that looked like confetti became one of the most horrific images of the terrorist attacks. “Perhaps they were jumping,” says Dallal. “I don’t know, and never will, nor will anyone. There is nothing even remotely romantic about that to my mind.”

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