A helicopter ferrying a patient with a medical emergency from the Grand Canyon collided with another chopper carrying a patient near a northern Arizona hospital, leaving six people dead and critically injuring a nurse.

The collision Sunday east of Flagstaff Medical Center was a few hundred yards away from a neighborhood that was spared the falling debris. Officials said they were unable to provide an account of what preceded the crash.

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National Transportation Safety Board spokeswoman Bridget Serchak said Monday that agency investigators were expected in Flagstaff at about 8 a.m. Arizona time.

Lawrence Garduno, who lives about a half mile from the crash, said he heard a loud boom that rattled the windows. He drove toward the hospital and stopped to see the burning wreckage. "It kind of scares me," Garduno said. "If this had happened a half mile closer, it could have fallen on our house."

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 were spending the night at the hospital, but their injuries were not life-threatening. The crash, about 130 miles north of Phoenix, also sparked a 10-acre brush fire that was contained.

One of the helicopters was operated by Air Methods from Englewood, Colo., 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.

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 suffered extensive injuries and was in critical condition at the hospital.

Matt Stein, a program director and lead pilot with Classic Helicopters subsidiary Classic Lifeguard Aeromedical Services in Page, Ariz., said his company's crew was landing at Flagstaff Medical Center carrying a patient with a medical emergency from the Grand Canyon's South Rim.

"We've been in business 20 years, and these are the first fatalities we've experienced," Stein said. "They were all heroes. They were out doing a great service for their communities."

Stein didn't tell The Associated Press the names of the crash victims, except to say that the pilot for Classic was experienced with more than 10,000 hours of flight time. He added that it's rare for two medical helicopters to attempt to land at a hospital at the same time.

Flagstaff Medical Center doesn't have flight controllers, he said, and it's up to the pilots to watch each other as they approach.

Aaron Todd, chief executive for Air Methods Corp., said Monday that his company's helicopter was being flown by a veteran pilot. Citing the ongoing investigation, he declined to discuss details about the pilot or the aircraft, which was placed into service in June 1998.

The helicopters spread debris across the scene. "They're not recognizable as helicopters," said Capt. Mark Johnson, a spokesman for the Flagstaff Fire Department.

The FAA is sending inspectors to investigate.

Hospital officials declined requests to interview the hospital president and the two burn victims.

It was the largest loss of life involving helicopters in Arizona since two news helicopters collided last summer while covering an auto chase near Phoenix, killing all four people on board.