HAWTHORNE, Nev. – A mortar shell explosion killed seven Marines and injured a half-dozen more during mountain warfare training in Nevada’s high desert, prompting the Pentagon to 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 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 mortar round exploded in its firing tube during the exercise, Brig. Gen. Jim Lukeman said. He said investigators are trying to determine the cause of the malfunction.

The Pentagon expanded a temporary ban to prohibit the military from firing any 60mm mortar rounds until the results of the investigation. The Marine Corps said Tuesday a “blanket suspension” of 60mm mortars and associated firing tubes is in effect.

The Pentagon earlier had suspended use of all high-explosive and illumination mortar rounds that were in the same manufacturing lots as ones fired in Nevada

It was not immediately clear whether more than a single round exploded, a Marine Corps official said, speaking on condition of anonymity because the official wasn’t authorized to speak about an ongoing investigation.

Eight men younger than 30 were taken to Renown Regional Medical Center in Reno with injuries, such as penetrating trauma, fractures and vascular injuries.

One of them died, five were in serious condition, one in was fair condition and another was discharged, spokesman Mark Earnest said.

The identities of those killed is being withheld pending family notification.

“We send our prayers and condolences to the families of Marines involved in this tragic incident,” said the force’s commander, Maj. Gen. Raymond Fox. “We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice.”

The rescue was complicated by the remoteness of the site, which is favored because the harsh geography simulates conditions in Afghanistan.

The 60mm mortar is a weapon that traditionally requires three to four Marines to operate, but it’s common during training for others to observe nearby.

The firing tube is supported in a tripod-like design and fires roughly a 3-pound shell, about 14 inches in length and a bit larger than 2 inches in diameter.

The mortar has changed little since World War II and remains one of the simplest weapons to operate, which is why it is found at the lowest level of infantry units, said Joseph Trevithick, a mortar expert.

“Basically, it’s still a pipe and it’s got a firing pin at the bottom,” Trevithick said. Still, a number of things could go wrong, from a fuse malfunctioning, a problem with the barrel’s assembly to a round prematurely detonating inside the tube, he said.