The Ground Self-Defense Force has failed to identify the cause of last year's deadly helicopter crash off Okinawa Prefecture's Miyako Island in an investigative report on the incident, Defense Ministry officials said Monday.

The GSDF will soon release the report on the crash of a UH-60JA helicopter near the island that killed all 10 people on board, including Yuichi Sakamoto, then head of its 8th Division.

An analysis of the flight data recorder recovered from the helicopter showed that the output of its right side engine had dropped rapidly shortly before the crash, according to the ministry officials.

This was apparently caused by a phenomenon known as rollback, where engine output drops gradually due to a pipe problem. As the output of the left side engine also decreased soon after that, the helicopter became unable to maintain its altitude and crashed.

The cause of the incident could not be pinpointed because part of the flight recorder data was damaged, making it impossible to determine whether the power loss of the left side engine was caused by a malfunction or pilot error, the officials said.

The GSDF plans to conduct thorough aircraft maintenance and pilot training to prevent any repeats of such an incident.

The helicopter crashed in the afternoon of April 6 last year after disappearing from radar during an aerial inspection of the terrain of Miyako Island, about 10 minutes after it took off.