Two American Black Hawk helicopters collided in midair and crashed in the northern city of Mosul Saturday. At least 17 U.S. soldiers were killed and 5 wounded in the deadliest single incident involving U.S. troops since the start of the Iraq war. The incident brings the death toll of US troops since the war began to over 400.

Military officials are still investigating reports that the two choppers collided when one ascended suddenly to avoid ground fire. Five U.S. helicopters have been downed by hostile fire in the past three weeks. Also in Mosul, five U.S. soldiers were injured when a roadside bomb detonated under their convoy.