CAMP PENDLETON -- The explosion that killed four enlisted Marines during a Nov. 13 training exercise was probably caused when a grenade round was “dropped, kicked or bumped,” according to a investigation report released Thursday.

The grenade set off an explosion among several hundred grenades and other shells that had been gathered for demolition.

As a result of the investigation, a captain and master sergeant were relieved of command -- an action that may end their careers.

Brig. Gen. John Bullard, commanding general of Marine Corps Installations West, said that the exact cause of the accident may never be known.


The four Marines were explosive ordnance disposal specialists who had been assigned to clear away thousands of rounds from one of the impact areas on the base.

The captain and master sergeant had failed to adequately assess the potential danger, the general concluded.

Ordnance specialists have a saying: “If you pick it up, you own it until you put it down,” Bullard said.

While it is unknown whether the grenade was dropped, Bullard said, the investigation showed that Marines were handing grenades to each other during the collection process, a technique that is not prohibited but heavily discouraged.


The 4-inch grenade round is designed to be launched from a ground- or vehicle-mounted position.

The investigation, and a parallel one by safety specialists, will probably result in new rules for handling unexploded ordnance on training fields, Bullard said.

The incident is also expected to act as a “wake-up call” to Marines assigned to ordnance disposal, he added.

Explosives disposal has been one of the most crucial and dangerous missions in Iraq and Afghanistan, where buried bombs have been used as a “weapon of choice” against U.S. and coalition troops. Disposal technicians are widely lauded by frontline troops.


“Their actions on the battlefield are absolutely heroic ... saving other Marines’ lives by rendering safe improvised explosive devices,” Bullard said.

All four Marines had deployed to combat zones and been awarded a Combat Action Ribbon, awarded only to Marines who were under fire and returned fire.

Those killed were:

Staff Sgt. Mathew Marsh, 28, of Long Beach. He deployed twice to Iraq and twice to Afghanistan.

Gunnery Sgt. Gregory Mullins, 31, of Bayou L’Ourse, La. He deployed twice to Afghanistan.

Sgt. Miguel Ortiz, 27, of Vista, Calif. He deployed to Iraq and Afghanistan.

Staff Sgt. Eric Summers, 32, of Poplar Bluff, Mo. He deployed to Iraq and three times to Afghanistan.

The four had a total of seven children, all under age 10. The Marine Corps Scholarship Foundation has awarded each child $30,000 in college scholarship support.


Two other Marines and a Navy corpsman were injured in the accident and were treated and released after receiving medical assistance at the scene.

The Marines had been assigned to clear away unexploded ordnance on the Zulu range on the sprawling base. Clearing away ordnance is a basic maintenance chore in preparation for using the range for training.

Part of any investigation into a training death is an examination of whether safety precautions were followed.

In March 2013, seven enlisted Marines from Camp Lejeune, N.C., were killed during mortar training at the Hawthorne Army Depot in Nevada.


The premature explosion of the 60-millimeter mortar round that killed the seven was later determined to have been caused by human error in using a mortar, the Marine Corps said.

A lieutenant colonel, a captain, and a chief warrant officer were relieved of command as a result of the investigation.

tony.perry@latimes.com

Twitter: @LATsandiego