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An explosion that killed four Marines at Camp Pendleton in California last year was probably caused when a grenade round was mishandled, the base said. Two Marines were relieved of duty after an investigation.

The base said that the exact cause would never be known because the only eyewitnesses were killed. The four Marines, aged 27 to 32, had all served tours in Iraq, Afghanistan or both. Three had at least a decade of service time.

They were killed during an exercise on ordnance disposal. The base said the grenade round was probably “dropped, kicked or bumped” in a demolition pit. Investigators believe it set off other explosives being gathered for disposal, NBC San Diego reported.

The two dismissed Marines, an officer and a staff non-commissioned officer, were relieved for a lack of confidence in their ability to conduct training and adhere to guidelines.

Brig. Gen. John Bullard, the base commander, ordered a review of training procedures. He said the Marine Corps was “deeply saddened” by the deaths.

“Their loss is felt throughout the Marine Corps,” he said. “We offer our heartfelt prayers and thoughts to the family members, and will continue to support them through this difficult time.”