The impact of the accident was immediately felt.
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 60 mm 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.
The 60 mm 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, some 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 with Global Security.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, such as a fuse malfunction, a problem with the barrel's assembly, or a round prematurely detonating inside the tube, he said.
A Marine Corps official said an explosion at the point of firing in a training exercise could kill or maim anyone in or near the protective mortar pit and could concussively detonate any mortars stored nearby in a phenomenon known as "sympathetic detonation." The official spoke on condition of anonymity because the individual wasn't authorized to speak about an ongoing investigation.
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 mortar shells manufactured at the same time as the one involved in the accident were safe.
The official said it would be normal to warn other U.S. military branches that use 60 mm mortars, such as the Army, 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, or whether there was a malfunction in the firing device or in the explosive mortar shell itself, the official said.
Renown hospital emergency physician Dr. Michael Morkin said at a news conference that some of the injured Marines he treated were conscious and "knew something happened but didn't know what." Morkin said the Marines mostly suffered blunt force trauma from shrapnel.
"They're injuries of varying severity ... to varying parts of the body. They're complicated injuries to deal with," he said.
The Hawthorne depot 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.
Bridis reported from Washington. Contributing to this report were Associated Press writers Pauline Jelinek in Washington, Allen Breed in Camp Lejeune, N.C., Julie Watson in San Diego, Martin Griffith in Reno, Nev., Michelle Rindels and Ken Ritter in Las Vegas, Joseph Altman in Phoenix and Mitch Stacy in Columbus, Ohio.
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