At least seven people were killed and three critically injured on Sunday when two medical helicopters collided over Flagstaff, Arizona, a Federal Aviation Administration spokesman said.
The mid-air collision occurred near a hospital in the city and at least one of the fatalities was a patient, FAA spokesman Ian Gregor said.
He said at least seven people were killed as a result of the mid-air collision, but it was not clear if that included any people on the ground.
Local KPHO television said the seven killed included a helicopter pilot and that three other people were critically injured in the accident.
The crash of the two choppers triggered a wildfire in a heavily wooded area that was being brought under control, a Flagstaff Fire Department spokesman told the television station.
FAA inspectors were heading to the scene of the accident, which occurred at 3:45 pm (1045 GMT), Gregor said.
"I certainly wouldn't want to speculate on what would have caused this," he said.
The helicopters, both Bell 407s, are used to transport patients from one hospital to another. Three of the fatalities were aboard a helicopter operated by a company called Air Methods. The other aircraft was run by Classic Helicopter Service of Utah.