"A fatal incident occurred at approximately 11 a.m. ... during a range maintenance operation," San Diego's KGTV-TV reported base officials said in a statement.
A Marine official told KABC-TV, Los Angeles, the Marines who died were handling unexploded ordnance at a firing range. They were part of a range maintenance team ordered to take care of any unexploded ordnance, the official told the TV station.
The names of the victims had not been released. No other Marines were reported injured, KGTV-TV said.
"We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident," said Brig. Gen. John W. Bullard, the camp's commanding general. "Our first priority is to provide the families with the support they need during this difficult time."