( CNN ) -- Four Marines were killed Wednesday during a range maintenance operation at Camp Pendleton in California , the military said . The incident was the deadliest for Pendleton since seven Marines were killed in a February 2012 midair collision between helicopters along the Arizona-California border , said a military spokesman . Six of them were based at Camp Pendleton , California , and the other was from Yuma , Arizona . Their names of the four Marines killed this week were n't immediately released because their families were still being notified , the Marines said . The four Marines were working in a range area used for live-fire exercises , but the deaths did n't occur under such live fire , said a military spokesman who asked not to be identified while the matter is under investigation . The incident could have `` very possibly '' involved unexploded ordnance suddenly detonating , the spokesman said . The range is known as Zulu Black , a Marine spokesman said . The deaths are under investigation , the Marines said . `` We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident . Our first priority is to provide the families with the support they need during this difficult time , '' Brig . Gen. John W. Bullard , commanding general of Camp Pendleton , said in a statement . In March , seven Marines were killed when a 60mm mortar round exploded at Hawthorne Army Depot in Nevada . An investigation revealed that `` the Marines employing one of the mortars did not follow correct procedures , resulting in the detonation of a high explosive round at the mortar position , '' the Marines said in a statement . Three Marines were relieved of their duties after the incident . The Marines said they were dismissed because their commanding general `` lost trust and confidence in their ability to ensure proper preparation for , and conduct of , live fire training events . '' 7 killed in explosion at Nevada Marine training exercise . CNN 's Barbara Starr and Miguel Marquez contributed to this report .
The four Marines were working on range area known as Zulu Black used for live-fire drills . But the deaths did n't occur under such live fire , a military source says . It 's possible that unexploded ordnance suddenly detonated , the source says . In March , seven Marines were killed when a mortar round exploded in Nevada .