By Marty Graham
SAN DIEGO (Reuters) - An explosion of live ordnance caused Wednesday's accidental deaths of four U.S. Marines while they were conducting routine maintenance in an artillery zone at Camp Pendleton in California, a base spokesman said on Thursday.
The type of shell that exploded on Wednesday morning at the Zulu Impact Area and how exactly it was detonated remain under investigation, the spokesman, Jeff Nyhart said.
The range, located deep in the interior of the sprawling base, is used as a target zone for artillery and aerial bombing practice, and requires routine maintenance to keep it clear of obstructions and unexploded rounds.
The four dead Marines were ordnance disposal technicians based at Camp Pendleton, not new recruits, and "were handling unexploded ordnance" when the blast occurred, Nyhart said. The victims' names and details of their unit were being withheld pending notification of their families, he said.
No live-fire exercises were under way at the site at the time of the accident, and no shells were being fired into the area from neighboring target ranges, he said.
More than a dozen Marines and sailors have died at Pendleton since 2004 in various mishaps ranging from helicopter crashes to accidental shootings, according to Los Angeles-based City News Service.
Seven Marines were killed in an accidental munitions depot blast in Nevada in March. In May, a U.S. Navy SEAL was killed in a vehicle accident during a training exercise at Fort Knox in Kentucky.
In September 2011, two Marines aboard a helicopter died when it crashed during a training mission at Camp Pendleton, two months after a Marine was killed when another helicopter went down at the base.
Camp Pendleton, 40 miles north of San Diego, is the main West Coast base for the Marine Corps. The base, with a land mass comparable to the size of Rhode Island, is home to 37,000 active duty Marines and sailors.
(Reporting by Marty Graham in San Diego; Writing and additional reporting by Steve Gorman in Los Angeles; Editing by Cynthia Johnston and Alden Bentley)