During a routine training exercise flight from Creech Air Force Base, northwest of Las Vegas, to Marine Corps Air Station Miramar in San Diego this week, a U.S. Marines helicopter crashed, leaving five Marines missing.

The five U.S. Marines who were aboard a military helicopter that went down in the mountains near San Diego were found dead, authorities said on Thursday.

The Marines flying a heavy-lift helicopter were conducting a “routine training flight” from Creech Air Force Base, northwest of Las Vegas, to Marine Corps Air Station Miramar in San Diego, when the aircraft was reported overdue on Tuesday, according to the 3rd Marine Aircraft Wing.

“It is with a heavy heart and a profound sadness that I share the loss of five outstanding Marines,” Maj. Gen. Michael Borgschulte, commanding general of the 3rd Marine Aircraft Wing, said in a statement. “To the families of our fallen Marines, we send our deepest condolences and commit to ensuring your support and care during this incredibly difficult time.”

The aircraft wing said efforts to recover the remains of the Marines and equipment has begun and an investigation is underway. The names of the Marines will not be released until 24 hours after all next-of-kin notifications are completed. It’s unclear where their bodies were found.

Local, state and federal authorities began a search and the aircraft was discovered in Pine Valley, California, about 45 miles east of San Diego, just after 9 a.m. on Wednesday. The condition of the aircraft has not been released nor the time the Marines departed from Nevada and when they were scheduled to land at the base in San Diego.