Authorities say a Colorado company operated one of two medical helicopters that collided in Arizona Sunday, killing six people on board and critically injuring another.

The Federal Aviation Administration says one helicopter was operated by Air Methods out of Englewood, and the other was from Classic Helicopters of Woods Cross, Utah.

After the collision, the helicopters crashed in a wooded area east of Flagstaff Medical Center and started a 10-acre brush fire. An explosion on one aircraft after the crash left two emergency workers who arrived with a ground ambulance company with minor burns.

The crash is under investigation.