WATCH: Haunting 9/11 Video Taken By NYPD Aviation Unit

It�s a heart-breaking bird�s eye view of New York�s darkest day.

As we mark the anniversary of the 9/11 attacks, a powerful 17-minute video taken from an NYPD helicopter on that fateful day surfaced on the Web a few years ago.

Obtained by the National Institute of Standards and Technology, the leaked footage from inside the chopper shows the twin towers engulfed in clouds of smoke � and captures the stunned reactions of the cops when they fell.

Earlier in the footage, the officers take in a panoramic view of the unfolding pandemonium � and get perilously close to the black roiling smoke.

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When the chopper briefly touches down in a nearby park, stunned officers gaze up at one of the burning towers. The video then shows the chopper back in the sky � and the aftermath of the now-fallen tower.