Unidentified officals cited by the CNN news network had earlier said that debris recovered on the ground indicated that a missile was involved.
Al-Qaeda in Iraq claimed on Monday that it had shot down the helicopter but a group called Jaish al-Mujaihadeen has also said it carried out the attack.
The crash came less than two months after a Sea Knight helicopter went down in the western Sunni Anbar province on December 3.
Four of the 16 military personnel on board were killed in the crash which occurred on the shore of Qadisiyah lake.
The worst US helicopter crash of the Iraq war was on November 15, 2003 and killed 17 soldiers when two UH-60 Blackhawks crashed west of Mosul in northern Iraq.
Two weeks earlier 16 US soldiers died when their CH-47 Chinook was shot down by a ground-to-air missile near Fallujah in Anbar province.