Investigators disassembled the muffler after the crash and found no problems that existed before the crash, according to the report. Likewise, the plane's airframe and engine showed no problems that could have caused the crash, according to the report.
An autopsy report released last year shows Olin Branstetter's cause of death as multiple blunt force injuries. A Federal Aviation Administration toxicology report shows no drugs or alcohol were detected in his muscle tissue.
Records show investigators didn't perform tests for cyanide or carbon monoxide poisoning. NTSB inspectors said it was impossible to perform those tests with the amount of tissue available.
Budke, 50, and Serna, 36, were heading from Stillwater to Little Rock for a recruiting trip. At the time, OSU officials exercised limited oversight in cases where donors offered to fly university personnel for free. A travel policy was in place for student athletes, but coaches and athletic staff were allowed a great amount of leeway in making their own travel arrangements.
OSU officials amended that policy last November to place greater restrictions on travel. Among other changes, the new policy ended the practice of allowing coaches to exercise their own discretion when making travel arrangements that don't include student athletes.
The new policy also requires that a university aviation consultant approve all private aircraft that would be used for university business, as well as the pilots who would fly them.
OSU officials said they overhauled the policy as a result of the crash, but didn't tailor it to the specific facts of the accident. Officials wouldn't speculate as to whether last year's fatal flight would have taken off had the new policy been in force. However, both Branstetter and the plane would have had to receive approval from an aviation consultant before flying.