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

The 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 Marine Expeditionary Force, based in Okinawa, Japan.

"It is believed that the aircraft ... was enveloped by rapidly developing clouds or lifted into a cloud by rising air currents. 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.

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.

Four of the Marines were part of the Marine Light Attack Helicopter Squadron 469 of the 3rd Marine Aircraft Wing based at Camp Pendleton near San Diego, California. Two other Marines were combat cameramen based in Japan.

The U.S. relief mission was deployed after a magnitude-7.8 earthquake struck Nepal on April 25, killing more than 8,200 people.