Investigators said Tuesday that a pilot's change of course shortly before his plane nose-dived into a Montana cemetery, killing all 14 aboard, has emerged as a potentially crucial factor in the crash.

Flying at 25,000 feet, pilot Buddy Summerfield requested the diversion from Bozeman to Butte half an hour before the single-engine Pilatus PC-12 crashed at the edge of Butte's airport Sunday. Seven children under the age of 10 were among the victims.

Click to view photos from the crash.

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

Summerfield said nothing to controllers to indicate he was having trouble, and did not say why he was changing course from Bozeman to Butte, about 75 miles away.

The probe into the crash will include a review of Summerfield's medical history, based on speculation that 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 that 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 the investigator in charge of the accident, Dennis Hogenson.

Also under scrutiny are weather conditions that could have caused icing on the plane's wings and possible overloading. The plane was configured to seat just 10 people, but the fact that several of the 14 passengers were small children has dampened speculation that 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 see if they could provide further clues.

Federal officials on Tuesday gave a few reporters and photographers their first close look at Sunday's crash site.

Working among rows of charred granite headstones cordoned off by yellow police tape, investigators removed the last of the victims' luggage as they combed through the wreckage. Shredded metal, pieces of propeller and a seat cushion were among the few discernible items left at the scene.

Since 2001, federal authorities have investigated 15 crashes involving PC-12's. Six involved fatalities, with a total of 14 killed prior to Sunday's crash. About 800 PC-12s are in service in the U.S.