COLLINSVILLE, Oklahoma - The National Transportation Safety Board says a plane crash that killed two men in Collinsville last year was caused by the pilot losing control of the aircraft, but it can't determine why that happened.

The Mooney M20J crashed into the backyard of a vacant house just west of downtown Collinsville at about 6 p.m. on April 7, 2013.

The plane was flown by 71-year-old Ron Marshall. He had a one passenger, 40-year-old Chris Gruber. Marshall, a retired doctor who specialized in obstetrics and gynecology, owned the single-engine plane. Gruber was Director of Development for Kansas State University's College of Veterinary Medicine.

In its Probable Cause Report on the crash, the NTSB says the cause of the crash was "The pilot’s loss of control of the airplane for reasons that could not be determined because an examination of the airplane did not find an abnormality that would have precluded normal operations."

In its previous report on the crash, the NTSB said an autopsy was not conducted on the pilot's body because of extensive trauma. For that reason, the NTSB says it could not determine if a medical or physiological issue contributed to the crash.

Even though the wreckage of the plane was consumed by fire, the NTSB says it did not find any problems that would have prevented it from operating normally before the crash happened.

Witnesses reported seeing a part of the airplane fall off just before it crashed and the NTSB confirmed that that did happen. However, in its final report on the crash the NTSB says the part likely fell off while the airplane was in a high-speed descent just before impact. In other words, the panel likely came off the airplane because it was already out of control.