The Aug. 13 crash of a CH-53D helicopter in Ginowan, Okinawa Prefecture, was due to a problem specific to just that chopper, a senior U.S. Marine Corps officer said Friday, defending the U.S. military's resumption of flights of the same type of aircraft.

The officer from the Third Marine Expeditionary Force in Okinawa, who briefed reporters on condition of anonymity, said U.S. investigators had found that the chopper lost tail rotor control due to the loss of a small retaining device in the assembly.

"We do not know why that part was missing, whether there was a material failure or whether the retaining pin was not installed properly," he said, noting the investigation is still ongoing into why the part was lost. "We just don't know at this point."

A full investigative report is expected to be submitted to the Japanese government in about a month.

The helicopter crashed Aug. 13 on a university campus near the U.S. Marine Corps Futenma Air Station during a training flight, injuring the three crew members on board.

CH-53Ds at Futenma resumed flights Sunday as part of the deployment of the 31st Marine Expeditionary Unit in Okinawa on a mission as part of the "global war on terrorism," according to the officer.

"The decision to return CH-53Ds to flight operations was not one made in haste," he said. Flights were resumed only after investigators determined that the cause of the accident was specific to the helicopter involved, the officer said.

The officer said there have been no flights of the same model since Sunday, although a squad of CH-53Ds will return to its home base in Iwakuni, Yamaguchi Prefecture, after essential safety inspections.