WASHINGTON (AP) — The deadly crash of a Marine V-22 Osprey in California last year was caused by mechanical failure, according to an investigation that ruled out pilot and maintenance errors.

The more than 400-page report released on Friday concluded that the Marines were doing routine flight operations when a “catastrophic, unpreventable and unanticipated mechanical failure occurred.” The Osprey crashed in a remote area near Glamis, about 115 miles (185 kilometers) east of San Diego.

Five Marines died in the crash: Two pilots, Capt. Nicholas P. Losapio, 31, of Rockingham, New Hampshire, and Capt. John J. Sax, 33, of Placer, California; and three crew chiefs, Cpl. Nathan E. Carlson, 21, of Winnebago, Illinois; Cpl. Seth D. Rasmuson, 21, of Johnson, Wyoming, and Lance Cpl. Evan A. Strickland, 19, of Valencia, New Mexico.

“The tragedy of this event is impossible to capture in words,” said Maj. Gen. Bradford Gering, who was commander of the 3rd Marine Aircraft Wing, in a memo attached to the report. “It is clear from the investigation that there was nothing the crew of SWIFT 11 could have done to anticipate or prevent this aviation mishap.”

The report also ruled out any issues with weather, birds or other external factors. And it concluded that there should be no disciplinary actions or administrative actions against any Marines.

According to the investigation, the specific cause of the crash was a “dual hard clutch engagement” that led to engine failure. The report said the V-22 program office has worked to fix the clutch problems.

Col. Brian Taylor, a program manager, said while the root cause of the clutch failure hasn’t been identified, a number of changes and equipment replacements have reduced the risk of it recurring by 99%, but have not eliminated it.

The report notes there were no direct witnesses to the accident in June 2022, and due to the fiery crash, the data recorder was not recovered.

As of April 2022, the Marine Corps and Navy had logged a total of 422,165 flight hours on the Osprey since 2012. The report said there have been five fatal crashes of Marine Ospreys since 2012, causing a total of 16 deaths.

The investigation also found that since 2010 there have been 16 similar clutch problems with the Marine Ospreys in flight. In February 2023 the Marine Corps began replacing a piece of equipment on the aircraft, and there have been no similar problems since then.

The Osprey is a hybrid aircraft that takes off and lands like a helicopter, but during flight it can rotate its propellers to a horizontal position and cruise like an airplane. Versions of the aircraft are flown by the Marine Corps, Navy and Air Force.