The only known aerial photographs of the World Trade Centre as it collapsed on September 11, 2001, were released by ABC News.

The photos, taken from a helicopter by New York Police Det. Greg Semendinger, show the twin towers in flames, falling amid huge, billowing clouds of dust, debris and smoke that envelop downtown Manhattan.

The 13 photographs, released on Wednesday, were among thousands of pictures that ABC News sought under the Freedom of Information Act from the National Institute of Standards and Technology, which investigated the collapse of the towers, agency spokesman Gale Porter said.

While other photographs of the falling towers were taken on September 11, 2001 from satellites, rooftops and other aircraft, these photos are the only ones known to have been taken up close from an aircraft flying overhead in city airspace.

Nearly 3,000 people died when the twin towers were attacked by hijacked airliners.

The aerial photographs had been partially published in the past but now are publicly available to researchers.

"With these pictures, you move from a specific slice of information to much greater context," said Jan Ramirez, chief curator at the National September 11 Memorial & Museum.