TOKYO — A Marine The crew's decision to use the most direct route to bring out the injured is the likely cause of a May U.S. military helicopter crash that killed 13 people during earthquake rescue operations in May in Nepal, the Marine Corps officials Corps said FridaySaturday.

The chosen route choice, which may have been made because one or more of the injured was in need of urgent treatment, took the UH-1Y Huey helicopter for a brief period over unfamiliar terrain in unstable weather, according to a news release from the 3rd III Marine Expeditionary Force, based in Okinawa, Japan.

"It is believed that the aircraft, call sign 'Vengeance 01,' was enveloped by rapidly developing clouds or lifted into a cloud by rising air currents," the news release read. "As they attempted to maneuver out of the weather conditions, they lost visual reference with the terrain and impacted the ground." the release read in part.

The six Marines killed in the crash were Capt. Dustin R. Lukasiewicz; Capt. Christopher L. Norgren; Sgt. Ward M. Johnson IV; Sgt. Eric M. Seaman; Cpl. Sara A. Medina; and Lance Cpl. Jacob A. Hug.

Lukasiewicz, Norgren, Johnson and Seaman Four of the Marines were assigned to part of the Marine Light Attack Helicopter Squadron 469 of the 3rd Marine Aircraft Wing based at Camp Pendleton near San Diego, California. Medina and Hug — a combat photographer and combat videographer — were assigned to Marine Corps Installations Pacific out of Okinawa, Japan.  Two other Marines were combat cameramen based in Japan.

Two Nepalese soldiers and five injured civilians were also aboard the downed helo.

It took three days to find the wreckage in mountainous terrain northeast of Kathmandu, the Nepalese capital.

All 13 on board died in the May 12 crash, including six Marines, two Nepalese soldiers and five injured civilians.

The crew was delivering humanitarian aid to remote areas of Nepal and evacuating casualties following a deadly U.S. relief mission was deployed after a 7.8 magnitude quake that struck the country Nepal on April 25, killing more than 8,200 people. The last radio communication with the aircraft was about 40 minutes before the crash, the release said, making it "impossible to know precisely what occurred or the factors that influenced the crew to fly this route."

The investigation concluded that mechanical malfunction or maintenance malpractices were not contributing factors to the mishap, according to the release. Marine officials stressed that the pilots and crew were experienced professionals, medically fit for flight duties and equipped with the most technologically advanced utility helicopter.

"We celebrate their lives by remembering their courageous and selfless actions in the wake of natural disaster," the release said. "The sacrifice they made for their respective countries will not be forgotten and our thoughts are and prayers remain with their families at this difficult time."