Two medical helicopters collided about a kilometer from a northern Arizona hospital, killing six people and critically injuring a nurse, a federal official said. Two emergency workers on the ground were injured after the crash. One of the helicopters was operated by Air Methods out of Englewood, Colorado, and the other was from Classic Helicopters of Woods Cross, Utah. Both aircraft were Bell 407 models, said Ian Gregor, a spokesman with the Federal Aviation Administration. After the collision Sunday, the helicopters crashed in a wooded area east of Flagstaff Medical Center and started a 10-acre (4-hectare) brush fire. An explosion on one of the aircraft after the crash injured two emergency workers who arrived with a ground ambulance company. They suffered minor burns and their injuries were not life-threatening, authorities said. "Crazy chaos, just lots of twisted metal wrapped up around people," Capt. Mark Johnson, a spokesman for the Flagstaff Fire Department, said near the crash site. Three people on the Air Methods aircraft, including the patient, died. On the Classic helicopter, the pilot, paramedic and patient all died. A flight nurse on the Classic helicopter was in critical condition at Flagstaff Medical Center.