The Pentagon halted the use of mortar shells pending an investigation after a shell exploded during training in Nevada, killing eight Marines and injuring seven.

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HAWTHORNE, Nevada — A mortar shell explosion killed eight U.S. Marines and injured seven more during mountain warfare training in the Nevada desert, prompting the Defense Department to halt the use of the weapons worldwide 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. The rescue of the wounded Marines was complicated by the remoteness of the site, which is favored because the harsh geography simulates conditions in Afghanistan.

The mortar round exploded in its firing tube during the exercise, said Brigadier General Jim Lukeman at a news conference in North Carolina, where the Marines are based. 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 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

8 Marines killed in Nevada…

8 Marines killed in Nevada training exercise

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.

The Marine Corps said early Tuesday that seven Marines were killed. Eight men under the age of 30 were taken to Renown Regional Medical Center in Reno. One of them died, four were in serious condition, two were in fair condition and another was discharged, said spokesman Mark Earnest.

John Stroud, national junior vice commander in chief for the Veterans of Foreign Wars, began a memorial event in Hawthorne on Tuesday night by saying “one of the critical has passed,” bringing the death toll to eight. Mourners then laid eight floral arrangements at a park where a flag flew at half-staff within sight of the Hawthorne depot’s boundary.

Stroud said he spoke with Marine officers from Camp Lejune who gave him the news before the ceremony. Messages left for a spokesman for the 2nd Marine Expeditionary Force were not immediately returned.

The force did issue a statement Tuesday evening saying an additional Marine has been reported as injured.

The identities of those killed won’t 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 2nd Marine Expeditionary Force commander, Maj. Gen. Raymond C. Fox. “We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice.”

The 60mm mortar traditionally requires three to four Marines to operate, but it’s common during training for others to observe nearby. The firing tube a shell some 14 inches (355 millimeters) in length.

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 with Global Security.org.

Still, a number of things could go wrong, including a fuse malfunctioning, a problem with the barrel’s assembly or a round prematurely detonating inside the tube, Trevithick said.

The Marine Corps official said an explosion at the point of firing in a training exercise could kill or maim anyone inside or nearby the protective mortar pit and could concussively detonate any mortars stored nearby in a phenomenon known as “sympathetic detonation.”

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 moratorium could last for weeks or months.

The Hawthorne Army Depot stores and disposes of ammunition. It 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.

Retired Nevada state archivist Guy Rocha said he was unaware of any other catastrophic event at the depot over the years it served as a munitions repository.

Associated Press writers Pauline Jelinek, Michelle Rindels and Ken Ritter contributed to this report.