"Preliminary investigations indicated that the med-evac helicopter was brought down by ground fire," said Brigadier General Mark Kimmitt, the coalition's deputy director of operations.
He added that "the investigation has not concluded".
The helicopter crashed on Thursday near the town of Falluja, west of Baghdad, an area which is a hotbed of resistance to the US occupation.
Witnesses said they had seen the helicopter in flames before it crashed.
Another US military officer, who declined to give his name, told AFP he had read a report indicating the pilot of a second helicopter in the area had seen ground fire which might have hit a tail rotor.
Of seven other US helicopters that have been lost as a result of hostile fire since the end of major combat operations in Iraq on 1 May, four were brought down by attackers in the Falluja area.
A Chinook helicopter was struck down by a surface-to-air missile on 2 November in the area, killing 16 people.