7 Marines killed in explosion during training exercise at Army depot in Nevada

Seven U.S. Marines were killed and at least seven wounded when a mortar exploded during a live-fire training exercise overnight at an Army munitions depot in the Nevada desert, military officials told NBC News.
There were conflicting reports about what happened. 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.
The accident happened just before 10...