Four U.S. Marines were killed at Camp Pendleton in San Diego County Wednesday morning during a range maintenance operation, base officials confirmed.



According to Camp Pendleton officials, the deadly incident happened around 11 a.m. At this point, few details have been released about what actually occurred.



NBC News reports that a U.S. military official said the incident involved the detonation of some unexploded ordnance on a range at the base. The Marines involved in the accident were Explosive Ordnance Disposal Marines, NBC News reported.



NBC News sources inside the Pentagon said the Marines were not clearing the range, but instead doing some sort of training when the deadly detonation occurred.



An official from the public affairs office told NBC 7 San Diego that the incident happened in the Zulu impact area, which is located roughly at the center of the base. The impact area is where Marines fire artillery and drop bombs from aircraft.



The official said there was artillery training on base Wednesday, but not in that specific area. He said there was no firing going on at the Zulu impact area at the time of the fatal incident.



The official also clarified what is meant by the term “range maintenance,” saying it’s a fairly broad term used to describe a variety of upkeep on those firing ranges and can include anything from mowing the grass to actually clearing artillery.



Officials said they are now in the process of notifying the families of the Marines killed on base. The names of the deceased Marines will be released once their families are notified.

Brig. Gen. John W. Bullard, Commanding General, Marine Corps Installations West - Marine Corps Base Camp Pendleton, released the following statement Wednesday:



"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."

The cause of the incident is under investigation.

This isn’t the first deadly training incident involving Camp Pendleton-based U.S. Marines.



This past February, a Camp Pendleton-based Marine died in an active-duty training exercise in Riverside County. The Marine was skydiving as part of training and went down near the Perris Valley Airport.



In December 2012, a 23-year-old U.S. Marine died in a car accident during a training exercise aboard Camp Pendleton.



In February 2012, a tragic helicopter collision killed seven U.S. Marines during a training exercise in Yuma, Ariz. Of those Marines killed in the chopper crash, six were from Camp Pendleton, while one was based at MCAS Yuma.



In September 2011, a helicopter crashed on the northeast corner of Camp Pendleton, killing two U.S. Marines and sparking a brush fire.



In March 2011, a 7-ton water truck rolled over during a Navy-Marine training exercise at Camp Pendleton, killing two Navy sailors.



In January 2011, a Battalion Commander was fired after a training accident involving an amphibious assault vehicle that sank to the bottom of a basin during a training exercise. One Marine was killed in the incident.