Two U.S. Marines were killed on a training mission Friday when their helicopter crashed in Southern California. Their names have not yet been released, and the cause of the crash is under investigation, according to a statement by Marine Corps Air Station Miramar.

The Third Marine Aircraft Wing chopper went down around 4:30 p.m. local time (4:30 p.m. EST) during a training exercise at the Marine Corps Air Ground Combat Center in Twentynine Palms, California, roughly 140 miles east of Los Angeles. The two Marines aboard succumbed to their injuries. Their names will be released after relatives are notified, the Marine Corps said in a statement.

“Our hearts go out to the families and friends of the two Marines we lost in this tragic accident,” the wing commanding general, Maj. Gen. Michael Rocco, said in the statement.

Details of the incident was not immediately available.