Horrifying new images have just been released of the 9/11 atrocity. They were taken by an NYPD detective from a helicopter flying above the city as the disaster unfolded

HE WAS cursed with a bird's-eye view of the biggest act of terrorism in US history. Hovering hundreds of feet above the Twin Towers, in an NYPD rescue helicopter, Detective Greg Semendinger had hoped to launch an audacious rooftop rescue, but all he could do was hit the shutter release and document the tsunami of dust and debris that rose as the towers fell on 11 September, 2001.

The stunning images, of the cloud settling over lower Manhattan after the collapse of the second World Trade Centre tower and the thick plumes of smoke and debris that envelop the centre after the first tower collapsed, were yesterday released for the first time after the American television network ABC secured thousands of new pictures from the government agency responsible for investigating the attacks.

ABC News obtained the images by filing a Freedom of Information Act request in 2009 with the National Institute of Standards and Technology (NIST), which had collected them from amateur, professional and freelance photographers as part of the investigation into the collapse of the towers.

After months of review and collection by NIST, ABC News was eventually provided with 2,779 pictures on nine CDs.

Some of the pictures have never been released before, such as those taken from NYPD helicopters showing the vastness of the dust cloud generated when the towers collapsed.

The photographer, Det Semendinger, was the first helicopter pilot on the scene. The Bell 412 helicopter had taken off from the Brooklyn headquarters of the New York Police Aviation Bureau and arrived at the Twin Towers just five minutes after American Airlines Flight 11 had, at 8:48 am, ploughed into the 93rd floor of the 110-storey North Tower.

With black smoke already streaming from the building, Det Semendinger was aware that hundreds of people, later estimates said 700, were trapped above the 93rd floor, including employees with Cantor Fitzgerald, the investment bank with offices on the 101st to 105th floor and the employees of the Windows on the World restaurant.

A second Bell 412, piloted by Detective Pat Walsh, also arrived on the scene and was less than 200 feet away when the second hijacked plane, belonging to United Airlines, slammed into the south tower.

Det Semendinger was intent on staging a rooftop rescue, similar to one he and his four-man team had carried out in 1993 when al-Qaeda had first attempted to destroy the trade centre, with a bomb in the basement garage.

The attack killed six people, injured 1,042 and sent thick smoke up through the stairwells. Back then, Det Semendinger had hovered the helicopter and lowered two men by rope on to the roof who proceeded to hack down radio and television antennae to clear a suitable landing spot.

The crew members, having been alerted by radio traffic among emergency services, knew there were people stuck in the stairwells with medical problems. Over the course of an hour they rescued 28 people in three separate missions.

Yet in the months that followed Det Semendinger's successful rescue, an ugly feud broke out between the New York Police Department, which had helicopters, and the New York Fire Department, which did not, over who had control during emergencies. After the garage bombing, the Port Authority of New York and New Jersey, which owned the World Trade Centre, and the fire department made a deliberate decision not to plan for future helicopter rescues.

The New York City Fire Chiefs Association wrote to the mayor saying the rescue was a "cheap publicity stunt" and that the people saved were in no danger.

The agencies rejected recommendations from police pilots that an area of the North Tower's roof be kept clear for helicopter landings.

The antennas were put back up, and the Port Authority kept the two sets of heavy metal doors leading to the building's only roof exit tightly locked. While other buildings in Manhattan were required by New York law to allow rooftop access in case of emergency, the Trade Centre was exempt.

On the morning of 11 September, Det Semendinger was prepared to unravel a 250ft hoist, which had fold-out seats and could carry as many as ten survivors at a time. Although the smoke was billowing over on to the roof, there was still a portion clear, but it remained empty.

Inside the building a number of people had tried to reach the roof but failed. Stephen Roach, a bond broker with Cantor Fitzgerald, phoned his wife Isabel twice from the 105th floor of the North Tower.

In one message, left on her answer machine, colleagues could be heard shouting: "Try the roof!" But Stephen yelled back: "There's no way out." A number of people, including the figure who became known as "The Falling Man", chose to leap to their death.

Records of calls to 911 operators show that people on the top floors were seeking help at least until 10:12am, one hour and 24 minutes after the plane hit.

Det Semendinger later said the wind that clear autumnal morning did leave a corner of the tower relatively free of smoke, almost until the building collapsed.

He estimated that by using the hoist with folding seats rescuers could have saved as many as a few dozen people, but "there was nobody on the roof".