A National Transportation Safety Board report says investigators were unable to pinpoint the exact cause of the plane crash that killed Oklahoma State women's basketball coach Kurt Budke and three others.
The probable cause report, released Wednesday, says investigators could not determine why the pilot lost control of the aircraft during the Nov. 17, 2011, flight from Stillwater to Little Rock.
Eighty-two-year-old pilot Olin Branstetter was killed in the crash, along with Budke, OSU assistant coach Miranda Serna and Branstetter's wife, Paula Branstetter.
An earlier report found the plane had been inspected about a week before it crashed.
Investigators have said most of the plane's cockpit instruments were unreadable, unreliable or destroyed in the crash.