Japan's Air Self-Defense Force concludes that a fighter pilot who crashed off northern Japan in April failed to detect the jet's rapid descent because he was disoriented.
The state-of-the-art F-35A plunged into the ocean off Aomori Prefecture during a training mission, killing the pilot.
The ASDF's accident investigation committee released an investigation report on Friday.
The report analyzed the plane's flight route using data from other F-35As that took part in the training mission.
The report says the fighter jet descended at a speed of about 1,100 kilometers per hour from an altitude of more than 10,000 meters for roughly 30 seconds just before crashing.
It also says the plane began to descend to maintain a safe distance from US military aircraft that passed through the same airspace.
But the pilot apparently couldn't correctly determine the attitude of the aircraft and made a steep dive.
The pilot is believed to have crashed without knowing his plane was descending because of low nighttime visibility at and other factors.
The report also says that although the aircraft twice sounded an alarm warning of the danger of crashing, the pilot might not have been able to recover from the steep descent.
The ASDF says it is highly unlikely the accident was caused by mechanical failure and it resumed flight operations of the F-35As on August 1st.