Five US Marines were killed when their CH-53E Super Stallion helicopter crashed in a mountainous area of southern California during a training flight, the corps confirmed Thursday, according to Anadolu Agency.

"It is with a heavy heart and profound sadness that I share the loss of five outstanding Marines from 3d Marine Aircraft Wing and the 'Flying Tigers' while conducting a training flight last night," Maj. Gen. Michael J. Borgschulte, commanding general of the 3rd Marine Aircraft Wing, said in a statement.

The identities of the dead were not released due to a Pentagon policy that mandates the public be notified 24 hours after the next of kin.

The chopper was "reported overdue" Wednesday after departing from the Creech Air Force Base near Las Vegas, Nevada en route to Marine Corps Air Station Miramar in San Diego. It was located that afternoon by civilian authorities in a mountainous region to the east of the Miramar military installation known as Pine Valley.

US President Joe Biden said he is "heartbroken" by the deaths and said he is "deeply grateful to the San Diego County Sheriff’s Department, the U.S. Air Force Auxiliary-Civil Air Patrol, and other federal, state, and local agencies for their professionalism and dedication in supporting search and recovery efforts."

"Our service members represent the very best of our nation—and these five Marines were no exception," he said in a statement. "Today, as we mourn this profound loss, we honor their selfless service and ultimate sacrifice—and reaffirm the sacred obligation we bear to all those who wear the uniform and their families."

The Sikorsky CH-53E is a mainstay of the US Marine Corps, used primarily as a heavy-lift transport aircraft. It is used by the American and Japanese militaries and has been in service for more than four decades.