An al-Qaeda-backed group has released an Internet video of what it said was the shooting down of a US helicopter in Iraq this week, which killed seven military personnel.
But a senior US military officer said initial reports suggested mechanical failure was to blame for the helicopter crash, the sixth in Iraq in the past three weeks.
The video, posted on a website used by insurgents, showed an apparent missile hitting the helicopter. The footage also showed the aircraft pulling away while trailing smoke before being engulfed in flames and crashing behind distant trees.
"Early indications are this last, so the sixth in this pattern, is (due to) mechanical failure," Army Lt. Gen. Douglas Lute, director of operations for the US military's joint chiefs of staff, told reporters at the Pentagon.
"There are some eyewitness accounts that cause professional aviation officers to believe that it was ... most likely a mechanical malfunction," Lt Gen Lute said.
"I'd be very cautious about drawing conclusions from things that are posted on the Internet," he said.
The insurgent group - the Islamic State in Iraq - claimed responsibility on Wednesday for downing the helicopter in western Anbar province. US officials said then they were probing the crash of a Marine transport helicopter in Anbar.
The video's soundtrack carried militant chants and what appeared to be an old speech by al-Qaeda leader Osama bin Laden.