SAN DIEGO Four U.S. Marines were killed on Wednesday in an accident while clearing a training area of obstructions at Camp Pendleton, just north of San Diego, California, a military spokesman said. 
 
 The marines were killed at the Zulu Impact Area in the interior part of the base, said Marines spokesman Lieutenant Ryan Finnegan. The cause of the accident was under investigation. 
 
 Finnegan could not confirm an NBC News report that the incident involved the detonation of unexploded ordnance, but did say the type of operation the marines were handling could involve ordnance disposal. 
 
 "We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident," Brigadier General John W. Bullard, commanding general of the base, said in a statement. 
 
 "Our first priority is to provide the families with the support they need during this difficult time," Bullard said. 
 
 The military did not release ages or other details about the four marines killed. 
 
 (Reporting by Marty Graham in San Diego, writing by Alex Dobuzinskis; editing by Cynthia Johnston, G Crosse) 

 Lenny Ignelzi / AP Vehicles file through the main gate of Camp Pendleton, Calif., on Wednesday. 
 
 Four U.S. Marines were killed Wednesday when unexploded ordnance blew up after a training accident at Camp Pendleton, Calif., military officials said. 
 
 The deadly accident occurred during a "range maintenance operation," the Marine Corps said in a statement. It gave no details and said none of the victims would be identified pending notification of their families. 
 
 A U.S. military official told NBC News that the Marines had just finished taking part in exercises about 11 a.m. (2 p.m. ET) when unexploded ordnance detonated. A spokesman for the base told NBC San Diego that while the incident happened in an area where Marines fire artillery and drop bombs from aircraft, there was no firing going in that area at the time. 
 
 NBC San Diego: 4 Marines Killed at Camp Pendleton 
 
 "We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident," the Marine Corps statement said. 
 
 The accident is at least the second deadly training incident this year involving Camp Pendleton-based Marines, NBC San Diego reported. 
 
 In February, a Marine died in an active-duty training exercise in Riverside County. The Marine was skydiving as part of training and went down near Perris Valley Airport. 
 
 Camp Pendleton, in southern California about 40 miles north of San Diego, is the Marine Corps' main amphibious training base on the West Coast and is home to the I Marine Expeditionary Force. 
 
 Jim Miklaszewski and Courtney Kube of NBC News contributed to this report. 
 
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– Four Marines at California's Camp Pendleton were killed today in a training accident, reports Reuters. It's not clear exactly what happened, but NBC News reports that they were killed when an unexploded ordnance blew up. The Marines, who haven't been identified, had reportedly finished their drills and were clearing the training area when the accident occurred. It follows one involving live ammunition that killed seven Marines earlier this year in Nevada.