Almost all of them jumped alone, although eyewitnesses talked of a couple who held hands as they fell.

One woman, in a final act of modesty, appeared to be holding down her skirt. Others tried to make parachutes out of curtains or tablecloths, only to have them wrenched from their grip by the force of their descent.

The fall was said to take about ten seconds. It would vary according to the body position and how long it took to reach terminal velocity — around 125mph in most cases, but if someone fell head down with their body straight, as if in a dive, it could be 200mph.

Horror: A person falls to their death after jumping from the north tower following the audacious terror strike which shocked the world a decade ago

When they hit the pavement, their bodies were not so much broken as obliterated.

Nothing more graphically spells out the horror of the 9/11 attacks on the Twin Towers than the grainy pictures of those poor souls frozen in mid-air as they fell to their deaths, tumbling in all manner of positions, after choosing to escape the suffocating smoke and dust, the flames and the steel-bending heat in the highest floors of the World Trade Centre.

And yet, tragically, they are in many ways the forgotten victims of September 11. Even now, nobody knows for certain who they were or exactly how many they numbered. Perhaps worst of all, surprisingly few even want to know.

From the earliest days after the 9/11 attacks, the American establishment and the media showed an overwhelming reluctance to dwell on those who jumped or fell from the Twin Towers.

If this was simply down to qualms at being considered intrusive or voyeuristic when individuals in the most appalling circumstances chose in desperation to die very publicly, it would be understandable.

But there are other, more complicated, reasons. In the aftermath of this attack on America’s sovereign territory — a period of intense patriotism — some considered that to choose to die rather than be killed showed a lack of courage.

And in this country of intense religious fervour, many believe that to be a ‘jumper’ was to choose suicide rather than accept the fate of God — and suicide in whatever circumstances is considered shameful or, indeed, a sin that will send you to Hell.

At the office of the New York chief medical examiner, a spokesman said this week that they did not consider these people ‘jumpers’. She insisted they fell from the 1,350ft tall, 110-floor skyscrapers, for jumping would imply suicide.

‘Jumping indicates a choice, and these people did not have that choice,’ she said. ‘That is why the deaths were ruled homicide, because the actions of other people caused them to die. The force of explosion and the fire behind them forced them out of the windows.’

Terror: An estimated 200 people jumped to their deaths to avoid being killed by the fires on 9/11

For those who have discovered that their loved ones may have been among the estimated 200 or more who plunged to their deaths, this uncomfortable official reticence can only compound the suffering they have already endured.

University administrator Jack Gentul cannot possibly imagine his late wife’s torment before she died. Alayne Gentul, mother of two and the 44-year-old vice president of an investment company, was in the South Tower and had gone up to the 97th floor to help evacuate staff after the other tower was hit. In her final moments, she rang Jack to say in labouring breaths that smoke was coming into her room through vents.

‘She said “I’m scared”,’ he tells me quietly. ‘She wasn’t a person who got scared, and I said, “Honey, it’ll be all right, it’ll be all right, you’ll get down”.’

Alayne Gentul’s remains were found in the street outside the building across from the tower — sufficiently far from the rubble to suggest she had jumped. Mr Gentul, who has since remarried, is not convinced she took that option but is clearly irked that some believe jumping was some sort of cop-out.

‘She was a very practical person who would have done whatever she could to survive,’ he explains in a quiet voice. ‘But how can anyone know what one would do in a situation like that, having to choose how you go from this Earth?’

The notion that she jumped is, indeed, consoling to Mr Gentul in some ways, in that she exercised an element of control over her death.

‘Jumping is something you can choose to do,’ he says. ‘To be out of the smoke and the heat, to be out in the air, it must have felt like flying.’

On the clear, blue morning of 9/11, investment banker Richard Pecarello watched from his office on the other side of the river as the second plane hit. His fiancée Karen Juday was working as an administrator at bond traders Cantor Fitzgerald in the North Tower.

He tried to phone her but there was no answer, and for days and weeks after he looked at photographs on the internet and wondered if she had jumped. She was vain about her face and used anti-wrinkle cream, and he was certain she would have jumped rather than face the flames.

Mr Pecarello, 59, made contact with Associated Press photographer Richard Drew, who had captured images of many of the jumpers, and asked to look through his archives. He saw a couple of photographs of a woman in cream trousers and blue top which he is convinced were of Karen.

‘There was one of her standing in a window with flames behind her and one of her falling from the building,’ Mr Pecarello says. ‘It made me feel she didn’t suffer and that she chose death on her terms rather than letting them burn her up.’

He has no time for suggestions that she took the easy way out. ‘The people who died that day weren’t soldiers. They were everyday people — parents and housewives and brothers and sisters and children,’ he says in his gruff Brooklyn accent.

Horror: The U.S. authorities have shown no interest in discovering who decided to jump rather than wait to be killed by the fire that ripped through the World Trade Center

When he tried to show the photos to Karen’s staunchly Protestant family back in Indiana, they didn’t want to know. They go by the official version, that nobody jumped.

In fact, nobody liked talking about the jumpers.

Unofficial estimates put the number of jumpers at around 200, but it is impossible to say for certain because their bodies were indistinguishable from others after the collapse of the Towers. The official account is that nearly all 2,753 victims in the Twin Towers attack officially died from ‘blunt impact’ injuries.

Ten years on, more than 1,000 have yet to be identified from remains. They were vaporised in the inferno.

After the planes hit, raging fires pushed the temperatures to 1,000c, sufficient to weaken the skyscrapers’ steel frames.

The metal conducted the heat through the building at a terrifying speed and it reached the upper floors long before the flames did.

There were reports of people having to stand on desks because the floor became so hot.

Fire experts say people rarely throw themselves out of burning high-rises until they have exhausted every other option. Indeed, as survivors desperate for fresh, cool air crowded at the windows smashed open by the force of the planes’ impact, it is possible some of the ‘jumpers’ were actually pushed out in the crush.

The only research that comes close to being an official account is buried deep in an appendix of the huge report into why the towers collapsed, conducted by the National Institute for Standards and Technology.

Grim: 2,753 died from 'blunt impact injuries' on 9/11 and it is thought that around 200 people jumped to their deaths. They would have fallen for around 10 seconds

As part of its research into where the fire was at its most intense, NIST analysed camera footage and still photographs, and counted 104 jumpers, often recording the floor and exact window from which they left.

All but three leapt from the first building to be hit — the North Tower. The second plane struck the South Tower 16 minutes later but it collapsed first, giving occupants less time to react.

The first jumper is recorded plunging from the North Tower’s 149th window of the 93rd floor on the north face of the building at 8.51am, just over four minutes after it was hit by the first hijacked Boeing 757 between the 93rd and 99th floors.

Sometimes the fallers were separated by an interval of just a second. At one point nine people fell in six seconds from five adjacent windows; at another, 13 people fell in two minutes. Twenty minutes after the building was struck, two people fell simultaneously from the same window on the 95th floor.

At least four jumpers tried to climb to other windows for safety then lost their grip. One person climbed from the 93rd floor to the 92nd, clinging to the window’s edge before falling just one second after someone else plumetted from the same window — number 215 on the east face of the tower.

The early jumpers came from the crash zone where the plane entered the building — the offices of the insurance brokers Marsh & McLennan.

The last jumper fell just as the North Tower collapsed 102 minutes after the building had been hit. Photographer Richard Drew says he has a picture of this person clinging to some debris while falling.

What drove some to jump and others to remain? Those who were in the South Tower, just 120ft away, at the time — and managed to escape — had the clearest view and may provide the best insight.

Kelly Reyher watched from the South Tower’s 78th floor as people started to fall out of ‘the hole’ the aircraft had ripped in the North Tower. To him, they looked ‘completely confused’ rather than consciously deciding to end it all.

‘It looked like they were blinded by smoke and couldn’t breathe because their hands were over their faces,’ he says. ‘They would just walk to the edge where the jagged floor was and just fall out.’

Charred remains: A huge investigation was launched following the terror tragedy - but no one has ever investigated at Ground Zero who may have jumped

Six floors below Mr Reyher, James Logozzo watched with stunned colleagues from the Morgan Stanley boardroom. He recalled that it took three or four jumpers to flash past him before he realised they were people. Then a woman fell, lying flat on her back and staring upwards. ‘The look on her face was shock. She wasn’t screaming,’ he recalled. ‘It was slow motion. After she hit the ground, there was nothing left.’

For those down below, the bodies landed with sickening, almost explosive thuds. Many said it was raining bodies.

One fireman, Danny Suhr, was killed as he made his way to the South Tower after a jumper landed on him, ‘coming out of the sky like a torpedo’ and breaking his neck. Compounding the tragedy, the priest who gave him the last rites was later killed by falling debris.

When she learnt how Danny died, his childhood sweetheart Nancy thought: how horrendous for that poor person who had to choose to jump; at least Danny did not have to make that choice. At least she had a body, for Danny’s colleagues took him to hospital after he was hit.

It was a decision that saved their lives — they would otherwise have been in the tower when it collapsed.

Firefighter Maureen McArdle-Schulman says she felt like she was intruding on a sacrament as the bodies fell. She adds: ‘They were choosing to die and I was watching them and shouldn’t have been. So me and another guy turned away and looked at a wall and we could still hear them hit.’

Bill Feehan, the deputy chief of the fire department, screamed at a man filming jumpers with a video camera: ‘Don’t you have any human decency?’

Fire battalion chief Joseph Pfeifer put out a desperate plea on the North Tower’s public address system. ‘Please don’t jump. We’re coming up for you,’ he said, not realising that nobody was listening — the system had long since been destroyed.

Images of the falling bodies disturbed and appalled all who saw them. On the first anniversary of the tragedy, an exhibition showing a work called Tumbling Woman, a bronze sculpture by artist Eric Fischl, lasted just a week in New York’s Rockefeller Centre before it was closed following protests and even bomb threats.

Human tragedy: Someone leaps from the burning World Trade Center on 9/11. It is thought that jumpers would have fallen for around 10 seconds

But one picture has become an iconic image. When a man fell at 9.41am from near the top of the North Tower, Richard Drew caught a dozen frames of his descent, including one in which he is diving vertically, arms by his sides and left leg bent at the knee. The image, all the more horrific for its desolate stillness, appeared the next day in newspapers around the world.

Dubbed the Falling Man, it prompted the media to hunt for the man’s identity. None of those who jumped from the towers has ever been officially identified and, tellingly, nobody rushed to claim Falling Man as their own.

Dark-skinned, goatee-bearded, wearing an orange T-shirt under a white shirt , he was first thought to be Norberto Hernandez, a pastry chef at the restaurant Windows on the World, on the top floors of the North Tower. His deeply religious family angrily rejected the notion, insisting that for him to have jumped would have amounted to a betrayal.

‘He was trying to come home to us and he knew he wasn’t going to make it by jumping out a window,’ his daughter Catherine says.

Grim: United Airlines Flight 175 collides into the south tower of the World Trade Center

Since then, the hunt for the Falling Man has moved on to another of the restaurant’s staff, Jonathan Briley, a 43-year-old sound engineer. The reaction of his deeply religious family has highlighted the deep moral complexities that suicide — whatever the circumstances — poses in a country where so many believe it is a sin, unforgivable by God.

Some of Mr Briley’s family have never believed he jumped, and say they were vindicated after the authorities found his largely intact body.

‘I had no idea it would give me the peace years later to know that,’ says his sister Gwendolyn. ‘If he had fallen from the 110th floor to the ground we wouldn’t have had that.’

Investment banker Richard Pecarello, 59, who tracked down that picture of his fiancee as she fell, also found peace. But for him it was in knowing that his fiancée did choose to jump. Most families have recovered no more than a fragment of bone, identified through DNA, of their loved ones, Mr Pecarello points out.

‘To me, the photo of her falling was like finding the body,’ he says. ‘I thought it was something that would help me move on. I needed to know how she died.’

When a 9/11 Memorial Museum opens at Ground Zero next year, it will have a small display dedicated to the jumpers, but reflecting the intense feelings of unease the subject has provoked, it will be tucked away in an alcove, on the grounds that the images are considered too private and too distressing.

It seems a harsh fate for those agonised mortals who faced the naked terror of that ten-second plunge to certain death. For the jumpers saved lives even as they were losing theirs.