A range maintenance operation went wrong at Camp Pendleton on Wednesday, when four U.S. Marines were killed. The California base is investigating the incident but did not provide any other details or names of the dead pending notification of family. "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," said Brig. Gen. John W. Bullard. Less than a year ago, seven Marines were killed in Nevada by a mortar explosion during a live-fire training exercise.