NEW YORK - A trove of aerial photographs of the collapsing World Trade Center was widely released this week, offering a rare and chilling view from the heavens of the burning twin towers and the apocalyptic shroud of smoke and dust that settled over the city.

The images were taken from a police helicopter - the only photographers allowed in the airspace near the skyscrapers on Sept. 11, 2001. They were obtained by ABC after it filed a Freedom of Information Act request last year with the National Institute of Standards and Technology, the federal agency that investigated the collapse.

The chief curator of the planned Sept. 11 museum pronounced the pictures "a phenomenal body of work."

The photos are "absolutely core to understanding the visual phenomena of what was happening," said Jan Ramirez of the National September 11 Memorial & Museum. They are "some of the most exceptional images in the world, I think, of this event."

In some of the pictures, the tops of the nearby Woolworth Building and other skyscrapers can just be seen above the enormous cloud of debris, gray against a clear blue sky. Gray clouds billow through the streets of the financial district and shroud the 16 acres where the towers had stood just moments before.

Buildings can hardly be seen at all in one image - just dust clouds hanging over the Hudson River at the southern tip of Manhattan.

One close-up shows orange flames and black smoke pouring from the upper floors of the north tower, the first hit by a hijacked plane.

"I almost didn't realize what I was seeing that day," Greg Semendinger, the former New York Police Department detective who took the 12 pictures posted on ABC's site, told The Associated Press. "Looking at it now it's amazing I took those pictures. The images are ... stunning."

The attack and the collapse of the World Trade Center were well documented on live TV and amateur video. But more than eight years after the nation's deadliest terror attack, the images still had the power to shock and disturb. They were an instant sensation on the Internet.

"Some survivors may find these pictures too painful to look at," said Richard Zimbler, president of the WTC Survivors Network. "But they are an important part of the historical record."

ABC said NIST gave the network 2,779 pictures on nine CDs. The network posted 12 pictures on its Web site Monday. ABC initially said some of the photographs posted had never been seen before, but later backed off that assertion

Semendinger said Wednesday that he had previously e-mailed some of the pictures to friends who later posted them on the Internet. Also, nine of the images were published in a book called "Above Hallowed Ground: A Photographic Record of Sept. 11" without his consent. The book was a tribute to the officers who were killed that day.

Semendinger was first in the air in a search for survivors on the rooftop. He said he and his pilot watched the second plane hit the south tower from the helicopter.

"We didn't find one single person. It was surreal," he said. "There was no sound. No sound whatsoever, but the noise of the radio and the helicopter. I just kept taking pictures."

He took three rolls of film with his Minolta camera, plus 245 digital shots. Semendinger said that he gave the digital images to the 9/11 Commission that investigated the attack, and that the commission evidently released the pictures to NIST.

Glenn Corbett, a fire science expert who sat on an advisory committee during the NIST probe, said the photos did not yield any new information for investigators.

"I don't see anything here that's new," he said. "These are common photos. ... It just reinforces things we know, that debris spread over a large area and the resultant dispersion of toxins and human remains."

Ramirez said the museum, slated to open in 2012, saw a selection of the photos at police headquarters several years ago. They are extremely important because the NYPD helicopter had the clearance to be up in the air in lower Manhattan only moments after the first tower was hit.