Federal investigators were unable to determine the cause of a fatal small-plane crash that killed Dave Rathbun, 52, of Hermantown, on Feb. 24, 2023, when the aircraft he was piloting crashed into the frozen St. Louis River near Grassy Point in West Duluth.

In its final report last month, the National Transportation Safety Board said Rathbun’s 2016 Cirrus SR22 rapidly descended before striking the frozen river nose-down “for reasons that could not be determined based on the available evidence.”

Investigation of the wreckage revealed no “preimpact mechanical malfunctions,” and the autopsy, hampered by the severity of his injuries, found “mild to moderate” narrowing of coronary arteries by plaque but “no other significant natural disease was identified,” the report said.

The autopsy on Rathbun’s body determined the cause of death to be “multiple blunt force injuries” and that it was an “accident.”

Rathbun, an engineer at Cirrus Aircraft for 26 years, was the plane’s only occupant.

According to the NTSB, Rathbun took off from the Duluth International Airport to reposition his plane to the Richard I. Bong Airport in Superior, Wis., where it was stored.

It was flying at an altitude of 1,300 feet on a 4-mile approach for runway 14 at the Bong Airport when it “suddenly pitched down about” 30 degrees and crashed into the river, leaving a 300-foot trail of debris, the NTSB said. The crash happened at 4:07 p.m. — just 4 minutes after he took off.

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