DULUTH – A small plane that crashed near Grassy Point on the St. Louis River in February 2023, killing the pilot, had a "rapid descent and subsequent collision with terrain," but investigators from National Transportation Safety Board can't say why it happened.

There was no evidence of a pre-impact mechanical malfunction, the NTSB said in its completed report. Likewise, the autopsy for pilot David Rathbun was inconclusive. It was limited by the extent of his injuries, which made it difficult to tell whether a medical condition led to the crash. His cause of death is listed as "multiple blunt force injuries."

Rathbun, of Hermantown, co-owned the plane, a Cirrus Design Corp SR22, and had taken it out to move it from the Duluth International Airport to the Richard I. Bong Airport in Superior, Wis., where it was based. The crash occurred about 4 minutes after takeoff. It was a clear day, late afternoon, with barely a hint of wind. The plane suddenly pitched down about 30 degrees, according to the report, then dove into the frozen St. Louis River.

Rathbun, 52, was alone in the four-seater that was built in 2016. The plane had been inspected within the past year.

Rathbun, a native of Kansas, had worked as a pilot and engineer at Cirrus for 26 years, according to his obituary.