PHOENIX -- 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."

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.

Six people aboard the helicopters died in the crash in Flagstaff, and one flight nurse was critically injured.

All three people on one of the helicopters operated by Air Methods of Englewood, Colo., were killed in Sunday afternoon's accident, including a patient, said Ian Gregor, a spokesman with the Federal Aviation Administration.

A pilot, paramedic and patient aboard the other aircraft operated by Classic Helicopter Corporation of Woods Cross, Utah, died in the crash, officials said. The flight nurse suffered extensive injuries and was rushed to Flagstaff Medical Center.

The three people who died on the Air Methods aircraft 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 remained in critical condition Monday at Flagstaff Medical Center.

Both aircraft were Bell 407 models, Gregor said.

Two personnel with an area ambulance service arriving on the scene were injured when one of the helicopters exploded after the crash, said Capt. Mark Johnson with the Flagstaff Fire Department.

"The EMTs showed them up," said John Kincaid, a witness. "I called them over and they asked me to back off. A few seconds after that was when one of the major explosions happened."

The crash happened around 3:45 p.m. in a heavily wooded area about a half-mile from the medical center, a Flagstaff Fire Department spokesman said.

The Flagstaff Fire Department quickly contained a 10-acre brush fire ignited by the crash.

An investigation is under way involving multiple agencies.

"The NTSB and FAA are involved as well as our own investigators," said Tom Boughner with the Flagstaff Police Department.

National Transportation Safety Commission personnel were due on the scene to spearhead the investigation.

Flagstaff police detectives said they need help identifying those who may have information about the crash. They urge people to contact the Flagstaff Police Department at 928-556-2300 if they can offer assistance.

Air Methods Corporation was established in Colorado in 1980 and serves as the largest provider of air medical emergency transport services and systems throughout the U.S., according to its Web site.

Classic Helicopters Limited, L.C. has a certified and trained medical staff to accompany the air ambulance helicopter into the air to provide for immediate emergency care, the company's Web site said.

This was the second medical helicopter crash in the past two days in Arizona. On Friday, a helicopter with three people aboard crash-landed in Ash Fork as it was picking up a patient.

A paramedic was the most seriously hurt. A flight nurse is in serious condition and the pilot is expected to be released from the hospital soon.

Last summer, two news helicopters collided while covering an auto chase in Phoenix, killing all four people on board.

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