A U.S. Marine Corps CH-53E helicopter of the same type involved in two accidents in Okinawa Prefecture last year crashed Tuesday in California, leaving the entire crew of four presumed dead.

The chopper went down near El Centro, California, Tuesday afternoon during what the Marines termed was a “routine training mission.”

The four crew members are presumed dead pending positive identification, The San Diego Union-Tribune quoted Marine spokeswoman Capt. Morgan Frazer as saying.

Authorities are investigating the cause of the crash. The names of the dead were withheld pending notification of their families, which typically takes about 24 hours.

Last October, a CH-53E helicopter from the 1st Marine Aircraft Wing caught fire in midair during a training flight and burst into flames as it made an emergency landing near the U.S. Northern Training Area on the main island of Okinawa. None of its seven crew members or local residents was hurt, but the Japanese Defense Ministry’s local bureau said the accident site was found to have been about 300 meters away from residential houses.

In December, a window from a Marine Corps CH-53E fell from the chopper, landing in a field where schoolchildren were playing.

The latter two incidents were the latest in a string of U.S. military aircraft mishaps on Okinawa, which hosts the bulk of U.S. military facilities in Japan. The U.S. presence there has long been a source of tension between the prefectural and central governments, and accidents and crimes involving the American military have angered local residents.