"The whole tower, it's gone," one officer is heard yelling.
"Holy crap, they knocked the whole fricking thing down."
An officer wonders, "How could it go down?"
The video is part of a cache of information from the attack handed over by city agencies to the National Institute of Standards and Technology, the federal agency that investigated the collapse.
The video surfaced on several websites Monday, but NIST did not know who posted it initially.
The 17 minutes of footage shot from a New York Police Department air and sea rescue chopper shows much of what has already been seen but still shocks and disturbs: a chilling aerial view of the burning twin towers and the apocalyptic shroud of smoke and dust that settled over the city.
Only police helicopters were allowed in the airspace near the skyscrapers, and the officers were the only ones shooting images from above. The helicopter flies over the roof as huge gray clouds billow, and away as the video pans out to lower Manhattan. A sea of people can be seen fleeing the area on clear, sunny day clouded with billowing smoke from the towers.
The chopper lands eventually across the harbor from the towers and the camera pans into the helicopter, showing ropes that would have been used to rescue people from the roof. The sparse dialogue portrays shock.
"We got out of there at the right time," one officer can be heard saying.
"I know," another officer replies.
The chopper crew watches in the distance as the North Tower falls, the video zooming to capture the image of the building going down and a huge plume of smoke puffing up.
"Holy s---," another officer can be heard saying.
Stunning, still images from police helicopters were released to the public last year under a similar request.
The video was released by NIST on March 3 under a Freedom of Information Act request, but it wasn't clear who published the footage online.
NIST investigated the collapse of the twin towers and another building that was part of the World Trade Center complex after the 2001 terror attacks.