Hostile fire
Military offcials said on Wednesday that an investigation showed the crash was "the result of anti-aircraft munitions".
Jeff Pool, a US major, said: "Initial evidence indicated that the CH-46 Sea Knight helicopter went down as a result of mechanical failure.
"After further investigation using all available means, the cause of the incident has been confirmed to be hostile fire."
The statement said the pilots of an AH-1W Super Cobra helicopter escorting the Sea Knight did not witness the actual attack, but they saw the fire, descent and subsequent crash.
It said the initial signs resembled fires that have occurred aboard CH-46s experiencing mechanical difficulties in the past.
The military said the crash site had been cleared with explosives after the remains and equipment were removed "to ensure the enemy could yield no gain from the debris".
Tactics changed
At least seven US helicopters have crashed or been forced down under hostile fire since January 20.
In the wake of the recent crashes, US officials have said they were reviewing flight operations and tactics but maintain there is no evidence of sophisticated new weapons used in any of the latest attacks.
On Friday, the Islamic State in Iraq, an umbrella group of Iraqi fighter groups that includes al-Qaeda in Iraq, claimed responsibility for downing the helicopter and issued an internet video it said was proof.
The group also claimed it shot down two other US helicopters.