The National Transportation Safety Commission will lead the multi-agency probe to try to determine why the two Bell 407 helicopters crashed, Phoenix television station KPHO reported.

A flight nurse aboard one of the helicopters was listed in critical condition in a Flagstaff, Ariz., hospital. Two personnel with an area ambulance service arriving on the scene were injured when one of the helicopters exploded after the crash, Flagstaff Fire Department Capt. Mark Johnson said.

The Sunday afternoon accident occurred in a heavily wooded area, the fire department said.

FAA spokesman Ian Gregor said at least one of those killed in the collision was a patient.

He said three of those killed, including the patient, were aboard a helicopter operated by Air Methods Corp. (NASDAQ:AIRM), an air medical service provider, CNN reported. The four other victims, including the critically injured nurse, were aboard the other helicopter, operated by Classic Helicopter Service of Utah.

Both helicopters were headed to the hospital at the time of the crash, Gregor said.