Report: Dropped or kicked grenade 'probable cause' of blast that killed 4 at Camp Pendleton

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A dropped or kicked grenade was the "most probable cause" of an explosion that killed four Marines during a training exercise last year at Camp Pendleton and a captain and master sergeant who were in charge were relieved of their duties, a base spokesman said Thursday.

The Nov. 13 blast occurred as ordnance disposal specialists were collecting unexploded rounds from a firing range at the San Diego County base for demolition — part of their training in the dangerous work of bomb disposal.

A Marine investigation report released Thursday indicated that a 40-mm grenade was accidentally "dropped, kicked or bumped," exploded and set off several dozen rounds that had been collected in a demolition pit, Capt. Ryan Welsh said.

However, the report listed the exact cause of the explosion as "indeterminate" and it may never be known.

"The only individuals with direct observations are deceased," Welsh said.

The captain and master sergeant who were conducting the ordnance disposal training were relieved of their duties on Tuesday — a move that potentially could end their careers.

The two lost their positions in charge of the base explosive ordnance disposal section "for loss of trust and confidence in their ability" to prepare for and conduct the training and for "lack of adherence to established norms," a base statement said.

For instance, they permitted the trainees to hand each other live ammo, Welsh said.

The explosion killed Staff Sgt. Mathew R. Marsh, 28, of Long Beach, Calif.; Sgt. Miguel Ortiz, 27, of Vista, Calif.; Gunnery Sgt. Gregory Mullins, 31, of Bayou L'Ourse, La., and Staff Sgt. Eric W. Summers, 32, of Poplar Bluff, Mo. All previously had served in Iraq, Afghanistan or in both countries.

A Navy Hospital Corpsman and two Marines near the accident had minor injuries.

In the wake of the explosion, Pendleton and other Western Marine bases were ordered to review and revise their explosive ordnance detail training, Welsh said.

Those bases are Marine Corps Air Station Miramar and Marine Corps Logistics Base Barstow, both in California, and Marine Corps Air Station Yuma in Arizona.