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 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 facility used by troops heading overseas, during an exercise involving the Second Marine Expeditionary Force from Camp Lejeune, N.C.
The mortar round exploded in its firing tube during the exercise, Brigadier General Jim Lukeman said at a news conference at Camp Lejeune. 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 that 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 under the age of 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, said spokesman Mark Earnest.
The identities of those killed will not be released until 24 hours after their families are notified.
‘‘We send our prayers and condolences to the families of Marines involved in this tragic incident,’’ said the force’s commander, Major General Raymond C. 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 is 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, some 14 inches in length.
The mortar has changed little since World War II and remains one of the simplest weapons to operate, said Joseph Trevithick, a mortar specialist with GlobalSecurity.org.
‘‘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.
The Marine Corps official said an explosion at the point of firing in a training exercise could kill or maim anyone inside or near the protective mortar pit and could concussively detonate any mortar shells stored nearby.
The official said a worldwide moratorium after such an accident is not unusual and would persist until the investigation determines that the weapon did not malfunction in ways that would hurt other Marines or that mortars manufactured at the same time as the one involved in the accident were safe.
The official said it would be normal to tell other military branches that use 60mm mortars about the Marines warning. The moratorium could last for weeks or months.
The investigation will focus on whether the Marines followed procedures to properly fire the weapon, whether there was a malfunction in the firing device or in the explosive mortar itself, the official said.
The Hawthorne Army Depot stores and disposes of ammunition.
The facility is made up of hundreds of buildings spread over more than 230 square miles, and bunkers dot the sagebrush-covered hills visible from the highway.
Hawthorne is in the shadow of Mount Baker, which reaches an elevation of 11,239 feet.
Retired Nevada state archivist Guy Rocha said the facility opened in 1930, four years after a lightning-sparked explosion virtually destroyed the Lake Denmark Naval Ammunition depot in northern New Jersey, about 40 miles west of New York City.
The blast and fires that raged for days heavily damaged the adjacent Picatinny Army Arsenal and surrounding communities, killing 21 people and seriously injuring more than 50 others.
Hawthorne has held an important place in American military history since WWII when it became the staging area for ammunition, bombs, and rockets for the war.
The Nevada Division of Environmental Protection says that the depot employed more than 5,500 people at its peak.