Japan's Air Self-Defense Force says it is highly likely a fighter pilot who crashed off central Japan in January became disoriented.
The F-15 jet fell into the Sea of Japan shortly after taking off from Komatsu Air Base in Ishikawa Prefecture. Two crew members on board were killed.
The ASDF's investigation committee released a report on the crash on Thursday.
The report says the fighter jet was turning to the right, while ascending. It says the aircraft was considerably tilting to the right and its nose gradually lowered before it started a rapid descent.
The report says that about two seconds before the crash, the pilot is believed to have tried to bring the jet back to its normal flight course, but failed to do so in time.
Noting the jet was flying in clouds at the time, the report concluded that the pilot was highly likely to have been suffering from "spatial disorientation," which can make it difficult to recognize which way is up.
The plane was on a drill to track another jet by radar. But shortly after take-off, the pilot reported he was unable to spot the dummy target plane. The report says another possibility is that the pilot was too focused on the radar.
The ASDF chief, General Izutsu Shunji, told a news conference that his organization will take steps to prevent a recurrence, and will study the system that automatically detects any abnormal position and alerts the pilots.