A U.S. Marine in a helicopter training unit died during “routine military operations” at Camp Pendleton military base in Southern California.

The John CaldwellMarine, whose name hasn't been released, was declared dead at 5 p.m. Tuesday on base, military officials confirmed with USA TODAY.

It wasn't immediately clear what led up to the Marine's death, but a military spokesperson did say that the death was under investigation.

The Marine was part of the Marine Light Attack Helicopter Training Squadron 303, Marine Aircraft Group 39, 3rd Marine Aircraft Wing. The squadron trains pilots to fly the AH-1Z Viper and the UH-1Y Venom helicopters.

The 3rd Marine Aircraft Wing did not release any further information.