Seven Marines were killed and seven wounded late Monday during a training exercise at the desert facilities of the Hawthorne Army Depot in Nevada, according to military and defense officials.
The Marines were part of the 2nd Marine Division, a ground combat force based at Camp Lejeune, N.C.
A senior defense official said it appears that a mortar round exploded inside a firing tube instead of flying out. The official, speaking on background, cautioned that initial reports could change on the cause.
The 230-square-mile facility, located about 140 miles southeast of Reno, stores conventional ammunition being taken out of use and, according to its website, also serves as "a premier training location for special forces."
"The unique aspects of mountainous terrain in a high desert environment provides a realistic simulation of the situation in Afghanistan," the website notes.
Army, Navy and Marine troops train year-round at the facility, which also provides training for foreign military nationals.
"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," said Maj. Gen. Raymond C. Fox, commanding general of the 2nd Marine Expeditionary Force. "We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."
An official at the 83-year-old facility tells KRNV TV in Reno that the explosion, which occurred just before 10 p.m. local time, was an accident unrelated to the ammunition stored at the depot.
Nevada Gov. Brian Sandoval said in a statement that he was "deeply saddened" to hear of the incident at the depot.
"The men and women who work and train there put service ahead of self each and every day," Sandoval said. "Kathleen and I wish to extend our deepest sympathies to those killed and their families. Our thoughts and prayers are with those who have been injured and we pray for their speedy recovery."
The U.S. Joint Munitions Command says on its website that the depot is an "archive site" for storing slow-moving ammunition and stocks awaiting disposal.
The facility has more than 2,915 structures including igloos, supply warehouses, maintenance buildings, munitions sheds, and office buildings.
Contributing: Associated Press