The fatal collision of two helicopters in Flagstaff was the ninth accident involving emergency medical aircraft this year, the chairman of the National Transportation Safety Board said Monday, calling it a "disturbing trend."

"This has been a serious issue. We're very concerned about that," NTSB chairman Mark Rosenker told reporters in Flagstaff. "We're going to work very, very hard to make sure we understand exactly what happened here, determine the probable cause and make recommendations to prevent it from happening again."

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Rosenker and NTSB investigators arrived in Flagstaff on Monday to begin probing the crash, a process that's expected to take several months to complete.

The helicopters crashed Sunday afternoon while ferrying patients to Flagstaff Medical Center, killing six people and critically injuring a nurse.

Rosenker says officials will look at similarities between the Flagstaff collision and one involving two news helicopters in Phoenix last year to identify any potential safety gaps. Four people died in the Phoenix crash last July.

NTSB spokesman Ted Lopatkiewicz said 16 people have died in EMS helicopter accidents so far this year. Helicopters accounted for six of the nine accidents of EMS aircraft this year that either caused serious injury to people on board or damage to the aircraft.

On Sunday, one helicopter was coming in from the South Rim of the Grand Canyon and another from the nearby community of Winslow, said Capt. Mark Johnson, a spokesman for the Flagstaff Fire Department.

After the crash, an unidentified bystander helped one person get out of one of the helicopters before emergency workers arrived and removed two others, Johnson said. It was unclear whether any of those people were the sole survivor.

No other details were immediately available.

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 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. They were identified Monday as pilot Pat Graham, 50, flight nurse Shawn Shreeve, 36, and patient Raymond Zest, 54.

On the Classic helicopter, pilot Tom Caldwell, 54, paramedic Tom Clausing, 36, and patient Michael McDonald, 26, all died.

A flight nurse, James Taylor, 36, on the Classic helicopter suffered extensive injuries and was in critical condition Monday at Flagstaff Medical Center.

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.

Stein said the pilot for Classic was experienced with more than 10,000 hours of flight time.

An Air Methods Corp executive, Craig Yale, said the pilot on his company's aircraft had 5,300 hours of flight experience.

He said the helicopter had been put into service in June 1998 and had no known issues.