The morning commuter flight was carrying a pilot and five passengers bound for Skagway when the aircraft reportedly veered right while accelerating on the runway.
NTSB says no mechanical faults that would explain the sudden loss of control that led to "substantial" damage to the aircraft.
NTSB's blames the Guardian Flight crash on: "A loss of control for reasons that could not be determined based on the available information." The three crew members were killed in 2019 after the twin-engine turboprop crashed into Frederick Sound during its approach into Kake.
NTSB investigators say they still don't know what caused the air ambulance to plunge 2,500 feet in about 14 seconds as it made its approach into Kake. All three crew members perished in the Jan. 29, 2019 crash over Frederick Sound.