CAMP PENDLETON, Calif. (AP) — An internal investigation says a deadly Marine helicopter crash in Southern California last year was likely caused by debris that hit the tail rotor, along with a design flaw and pilot error.

U-T San Diego obtained documents on the investigation into the July crash at Camp Pendleton that killed one Marine and injured five others.

The Marines found that debris such as a stray bolt damaged the tail rotor and the chopper's drive train was destroyed, causing a loss of tail rotor thrust. The pilot aggravated the situation by trying to move forward instead of reducing the throttle.

The investigation says that was a training problem and no administration action was recommended against the crew. It says the helicopter lacked protective barriers that might have prevented the debris damage.