A missing cotter pin caused the U.S. Marine Corps helicopter crash on a university campus near the Futenma Air Station in Okinawa in August, according to a U.S. investigation report submitted Tuesday that laid the blame on poor maintenance.

The pin was not installed in a connecting bolt on a subassembly of the CH-53D helicopter’s tail rotor, according to a statement released by U.S. Forces, Japan, after the day’s meeting of the fact-finding Subcommittee on Unusual Occurrences of the Japan-U.S Joint Committee.

The committee discusses matters concerning the bilateral Status of Forces Agreement.

The bolt fell out during flight, leading to a total loss of tail rotor control, it said, adding that pilot error was not a factor.

The statement also said maintenance procedures have been improved and that the U.S. military will carry out additional preflight checks of key parts. Those responsible for the accident will face disciplinary or administration action, according to the statement.

A Japanese official who briefed reporters on the day’s meeting said the ground crew had been working overtime and was under pressure to quickly prepare the helicopter for flight.

The two nations will draft a document of recommendations to prevent a recurrence and submit it to the Joint Committee.

The 200-page report was not revealed to the media. Both sides said they would consider disclosing the information at a later date, the official said.

Japanese helicopter experts, including Air Self-Defense Force officers, and committee members are expected to visit the U.S. Marine Corps Futenma Air Station later this week to view the crashed copter and hear explanations about maintenance procedures.

The U.S. side said Tuesday it hopes to soon resume CH-53D flights, which have been suspended since late August at the behest of the Japanese government, he said.

“The U.S. has investigated the cause of the accident and taken safety measures,” a U.S. official was quoted as saying. “We need to resume flights at a certain point.”

After meeting with Foreign Minister Nobutaka Machimura later Tuesday, Okinawa Gov. Keiichi Inamine said he doubts the report was compiled after a thorough investigation.

He said, “I wonder if the cause of the accident was only due to poor maintenance.”

The report should be made public, he said, adding, “The report should explain the accident in a way that convinces the public.”

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