A mortar shell explosion killed seven Marines and injured seven more during mountain warfare training in Nevada's high desert, prompting the Pentagon to immediately halt the use of the weapons until an investigation can determine their safety, officials said Tuesday. The explosion occurred Monday night at the Hawthorne Army Depot, a sprawling facility used by troops heading overseas, during an exercise involving the 2nd Marine Expeditionary Force from Camp Lejeune, N.C. Several Marines from the unit were injured in the blast, authorities said. The Marines were practicing firing mortars, reports CBS affiliate KTVN-TV in Reno, when one of the rounds exploded while still inside the tube. Brig. Gen. Jim Lukeman said investigators were trying to determine the cause of the malfunction. The Pentagon expanded a temporary ban to prohibit the military from firing any 60 mm mortar rounds until the results of the investigation. The Marine Corps said Tuesday a "blanket suspension" of