The helicopter was shot down as soon as it started its attack run, which is evident from the video.
It was hit by a weapon of unknown origin.
First they said it was TOW anti-tank missile, very unlikely except against stationary helicopter.
The second story circulated today is that it was a friendly rocket fire from a helicopter that was following behind.
Terrorist groups like to show a scene where a screaming terrorist (Allahu Akbaaaaar!!!) actually fires the missile. This wasn't show it this time, which makes their claim suspect.
Also, where is the missile trail. In past videos you can see the pursuing missile, every time, no exceptions.
"Artillery adds dignity to what would otherwise be a ugly brawl."
--Frederick II, King of Prussia