The brothers who made Ground Zero, the movie

Jules and Gedeon Naudet were struggling young film-makers. On 11 September, they happened to be filming in downtown Manhattan. Now their lives have changed forever. By David Usborne

It was serendipity that the French film-makers Jules and Gedeon Naudet were at the Duane Street firehouse in lower Manhattan good and early on 11 September. That they are both still alive is closer to a miracle. And something more survived: hours of footage they took of the swirl of destruction and death that day at the World Trade Centre.

The film shot by the Naudet brothers already has an almost mythic status. Unedited copies began circulating in FBI and firefighting circles a few weeks ago, like the samizdat writings of the Soviet Union. For a while we knew only this – whenever it finally emerged, it would be the only filmed record of those events from start to finish.

The mystery is over: CBS announced this week that it had acquired the rights to broadcast the footage. Now being polished in the network's own editing suites, the film will be shown, at last, on 10 March. It will be no ordinary night of television in America. CBS expects an audience of 50 million, more than watched last weekend's Super Bowl.

Already being dubbed the Zapruder of 11 September (after those famous frames of footage that captured the assassination of President Kennedy), the Naudet film is as yet unnamed. In fact, it is not about 11 September, as such. It is the result of a project that began months earlier and has only just ended. The pair were still filming this Monday.

Struggling to make their names as documentary film-makers, Jules and Gedeon, aged 28 and 31 respectively, had embarked in early 2001 on a piece about New York firefighters and, in particular, about the progress of one young rookie. He had been assigned to Engine 7, Ladder 1 on Duane Street, which happens to be near New York's financial district.

But the summer was immensely frustrating. They had permission from the fire department to ride with the firefighters to whatever disasters arose. They hung out at Duane Street whenever they could. But week after week, nothing happened. They yearned for at least one big fire. The poignancy of it is obvious now, of course. Little did they know.

Little did they know how they would, separately, capture not one but both of those hijacked airplanes ramming each of the twin towers. (One 10-second clip of the first plane penetrating the north tower was shown on US television in the week of the attacks.) Little did they know how they would both have to live through the worst kind of agony: each believing, for a while, that his brother must have been killed.

How the two of them travelled from professional despair to material overload in a few terrifying hours is told in next month's issue of Vanity Fair in an exclusive report written by editor David Friend, who, as it happens, has been close to the Gaudet family for years. The magazine, indeed, helped to ensure that the film was handled carefully and that CBS got the rights. Most of what it earns will be donated to a firefighters' scholarship fund.

"They are doing it so well and they are doing everything right," Mr Friend commented with evident pride this week (he has known the two brothers since they were tiny). "They wanted it to remain at a certain level of excellence, and they wanted to do something that would help raise money."

Their work is helping in other ways, too. The FBI has been poring over it to help in its investigations. Clips, meanwhile, will soon be provided to the families of firefighters who appear in the footage, rushing up stairwells to evacuate office workers, and who never came out alive. So far, about 90 dead men have been identified.

The day itself started with a call to the firehouse about a gas leak near the World Trade Centre. Jules rode in a van with the firehouse chief, Joseph Pfeifer, to check it out. He filmed the chief as he worked on the street. Then he heard the loud roar of what sounded like a very low-flying aircraft.

"I look up and it's clearly an American Airlines jet," he told Vanity Fair. "There is a pause of about two seconds before reality sinks in. Because I saw it disappear behind a building, I turn the camera toward where it is going – the World Trade Centre, directly down the block. I see it go in. I'm filming it go in."

From that moment, Jules, who until a few weeks earlier had worked as a producer and barely known how to use a camera, stuck close by the side of Chief Pfeifer, who set up a staging post for the deployment of his men inside the lobby of the north tower. His lens captured the frantic scenes inside. The chief sending his men up the tower, then desperately calling them down again. The two-way radios barely working. People jumping.

"Entering the lobby, I see two bodies burning on the floor. One is screaming, a woman's scream. The jet fuel has come down through the elevator shafts and created a fireball in the lobby. Time stands still. It is the first time I have seen someone about to die. The windows have all been blown out. The marble has come off the walls. In about five or 10 minutes, we hear what sound like explosions. A firefighter immediately says, 'We got jumpers.' People are leaping from such a height that, as they touch the ground, they disintegrate. Right in front of us, outside the lobby windows."

David Friend says watching the film now brings an expected sense of claustrophobia "because you feel you are in this space with them. You can't leave because Jules can't leave, because he is stuck with Pfeifer". And he is with Pfeifer when the other tower suddenly comes down.

"The light changes and everything becomes pitch black. I definitely think the building is coming down. I am afraid of dying. I put my T-shirt over my nose. The air is full of dust. I stop, crouch down, just waiting for the ceiling to crash on me. There is a strange calm." When the air clears, they see the body of the FDNY chaplain, Father Mychal Judge, on the floor. He is dead. "It was probably a heart attack," Jules says. "Or it could have been marble that flew off the wall."

Gedeon, meanwhile, was filming outside. "It is as if on D-Day you had someone with a camera with the commanders on the beach, and a second cameraman wading ashore with the grunts," says Mr Friend.

The elder of the two siblings could only think one thing. "As I am filming, I'm realising that all our firefighters are dead. And maybe Jules is dead." In fact, both brothers were still living. And they were still working. Their professional impulses, in fact, kept them calm. "I'm filming, because it is the only way to focus," Gedeon told Vanity Fair. "It is very important for me not to lose it, not to go hysterical. I don't want to think the worst about Jules."

When the second tower came down, both were outside. Gedeon leapt into a fire engine accompanied by an FBI agent. Jules, who was still far too close to the north tower for safety, fell to the ground under the weight of another man protecting him from falling debris, Chief Pfeifer. After that, Jules stopped filming and stumbled through the streets, worried now about only one thing – finding his brother. It wasanother two hours before both were back in the firehouse. They fell into each other's arms, tears smearing the dust on their cheeks.

In the days that followed, they had a choice – to release their footage to the networks immediately, or to persevere with their original project. Surprisingly, they took the second route. They spent three weeks at ground zero – all with the original blanket permission granted to them by the fire department – filming as the firefighters dug for the fallen. They went to Staten Island where the rubble was taken. And until Monday, they lingered at the firehouse doing final interviews with the survivors.

And there have been other miracles in this story. Nearly 350 firemen from New York died on that day. But everyone from Ladder 1, under the command of Chief Pfeifer, made it home alive. The firehouse lost no one. Although among those who did perish was Kevin Pfeifer, the chief's brother with another firefighting unit.

When the television special is edited and ready, there will be still more work to do. The brothers plan to re-cut the tape to make a version suitable for general release in cinemas across the US and around the world.

Jules, meanwhile, has to return to the top brass of the New York City Fire Department to ask one more favour of them. He and his girlfriend are to tie the knot this summer. He has a special venue in mind for the wedding; the firehouse of Engine 7, Ladder 1 on Duane Street.