BUTTE - Thirty minutes before a small plane crashed in Montana, killing all 14 aboard, the pilot requested a change in course from Bozeman to Butte. Why he did that is emerging as a key mystery.

Flying at 25,000 feet, California pilot Buddy Summerfield requested the diversion shortly before the single-engine Pilatus PC-12 crashed into a cemetery at the edge of Butte's airport Sunday, 75 miles away from Bozeman.

National Transportation Safety Board acting Chairman Mark Rosenker said finding out why the plane diverted is at the forefront of his agency's investigation.

"It begins with that question - the 25,000-foot diversion to go to Butte," Rosenker said.

Summerfield said nothing to controllers to indicate he was having trouble, and did not say why he was changing course from Bozeman to Butte.

The probe into the crash will include a review of Summerfield's medical history, based on speculation a heart attack or other health issue might have been at fault. Rosenker said air traffic controllers detected no strain in the 65-year-old pilot's voice during his final communications.

Rosenker also revealed the plane's landing gear was down, but its wing flaps were up at the time of the crash. That's unusual for a landing aircraft but not unheard of, said Dennis Hogenson, the lead accident investigator.

Also under scrutiny are weather conditions that could have caused icing on the plane's wings as well as possible overloading. The plane was configured to seat just 10 people, but the fact several of the 14 passengers were small children has dampened speculation excess weight was a factor.

The investigation has been hampered by the lack of a cockpit voice recorder or data recorder, which were not required on the private flight. Rosenker said his agency may subpoena cell phone records of the victims to look for further clues.

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