The Guardian explains:
Previously unseen footage of the 9/11 attacks, filmed from a police helicopter hovering above the burning World Trade Centre, has emerged almost a decade after the terrorist atrocity.
The New York Police Department air and sea rescue helicopter was dispatched to the scene of the attack to see whether any survivors could be rescued from the rooftops.
[. . .]
The video is part of a cache of information about the attack handed over by city agencies to the National Institute of Standards and Technology, the federal agency that investigated the collapse.
It was released by NIST on 3 March under a Freedom of Information Act request, but it remains unclear who published the footage online.
