Col. Joe Anderson said that though the investigation was not conclusive, it appeared that "some form of groundfire" -- likely an RPG -- hit one of the helicopters, sending it into the path of the other Black Hawk.

"Obviously it was a collision, and it appears that the collision was caused by one of them being hit by something from the ground, probably a rocket," said Anderson, whose 2nd Brigade of the 101st Airborne Division controls Mosul.