Military officials have confirmed that seven U.S. Marines were killed and many others were injured last night during a training exercise at Hawthorne Army Depot in Nevada.

NBC News reports, "a mortar exploded during a live-fire training exercise overnight... According to one account, a 60-millimeter mortar shell exploded in a tube as Marines were preparing to fire it. Another account said that the shell exploded as Marines were picking it up to load it."

Four of the Marines were apparently killed instantly while "two others died while waiting to be flown to the hospital," according to NBC News. The Hawthorne Army Depot website says it's "an archive site for storing slow-moving ammunition and stocks awaiting demilitarization." It's 140 miles southeast from Reno.

The incident is under investigation; Maj. Gen. Raymond C. Fox, commanding general of the 2nd Marine Expeditionary Force said, "We send our prayers and condolences to the families of Marines involved in this tragic incident. We remain focused on ensuring that they are supported through this difficult time. We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."