Federal investigators will be looking into the cause of a deadly helicopter crash in Arizona.

Six people, including two patients were killed and another critically injured when two medical helicopters collided in the air and crashed near Flagstaff Medical Center. At least one helicopter was approaching the hospital at the time. Two emergency workers on the ground also were hurt less seriously.

An official with one of the helicopter companies says the hospital doesn't have flight controllers, so pilots need to watch each other and communicate their locations by radio. He says it would be rare for two helicopters to be trying to land at a hospital at the same time.

A spokesman for the Flagstaff Fire Department says the wreckage was unrecognizable.