The United States military is today investigating whether ground fire caused the crash of two US helicopters, killing 17 American soldiers, the worst single loss of US life since the start of the Iraq war.
Five soldiers were injured and one was missing after yesterday's crash.
The two Black Hawks, which belonged to the 101st Airborne Division, went down in residential neighbourhoods of Mosul, Iraq's third-largest city.
The spokesman for the 101st Airborne, Major Trey Cate, said the military was "trying to figure out what happened.... We are going to do a thorough investigation because if this either involved ground fire or it was safety-related, then ... we're going to make sure we take precautions so it won't happen again."
The chief military spokesman in Baghdad, Col. William M. Darley, said the cause of the crash "will be under intense investigation today".
A statement by the US command said one helicopter was carrying a quick reaction force and the other ferried soldiers on a transport mission in northern Iraq. Cate said the quick response team was on its way to investigate a shooting incident in which a US soldier was injured.
The statement did not give the cause of the crash.
"The cause of the incidents are under investigation," the statement said.
An Iraqi policeman in Mosul, however, said at least one of the Black Hawks was hit by ground fire.
Another witness said she saw one helicopter above her house before hearing the sound of a loud boom. "I saw the sky light up, it was like thunder and lightning," she said, adding that after the initial boom she heard gunfire in the area but could not say where it came from.
One soldier at the scene told The Associated Press he heard that one of the helicopters was hit by a rocket-propelled grenade before it crashed. A US military spokesman said such reports were "at best speculative."