Story highlights The four Marines were working on range area known as Zulu Black used for live-fire drills

But the deaths didn'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

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 weren'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 didn'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."