New video of the September 11, 2001 attacks, shot from a police helicopter hovering near the burning World Trade Center towers, has been published on the internet.
The footage, filmed by police officers from the New York Police Department, was originally passed to the National Institute of Standards and Technology (NIST), the federal agency that investigated the collapse.
The video was released by NIST on March 3 under a Freedom of Information Act request, but it was not clear who published the footage online.
The video surfaced on several websites on Monday, but NIST did not know who posted it initially.
Only police helicopters were allowed in the airspace near the skyscrapers, and the officers were the only ones shooting images from above.
"The whole tower, it's gone," one officer is heard yelling.
The 17 minutes of footage shows much of what has already been seen but still reveals a chilling aerial view of the burning twin towers and the shroud of smoke and dust that settled over the city.