The Marines were clearing an artillery range of unexploded weapons, CBS reported. (US Navy/Creative Commons)

The cause of the accident is currently under investigation, and more information will be released as it becomes available. CBS reported that the Marines were clearing an artillery range of unexploded weapons, but it is not immediately clear when the accident happened.

The base said it would release the identities of those who were killed after their next of kin are notified.

"We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident," Brig. Gen. John W. Bullard, Commanding General of Marine Corps Installations West - Marine Corps Base Camp Pendleton said in the press release. "Our first priority is to provide the families with the support they need during this difficult time."

Seven Marines were killed in a mortar shell explosion in Nevada in March, which was later discovered to have been an accident of human error. 

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