The four Marines were working on range area known as Camp Bravo used for live - fire drills . But the explosion did n't occur under such live fire , a military source says . It 's possible that unexploded munitions suddenly detonated , the source says . Their names of the four Marines killed this week were n't immediately released because their families were still being notified , the Marines said . In March , seven Marines were killed when a mortar shell exploded in Nevada .
The four Marines were working on range area known as Camp Bravo used for live - fire drills . But the explosion did n't occur under such live fire , a military source says . It 's possible that unexploded munitions suddenly detonated , the source says . In March , seven Marines were killed when a mortar shell exploded in Nevada .