A mortar shell explosion killed seven US Marines and injured a half-dozen more during a training exercise in the Nevada desert.
The explosion occurred on Monday night at the Hawthorne Army Depot, a facility used by troops heading overseas.
It was not immediately clear whether the 60mm mortar shell exploded prematurely inside its firing tube or whether more than a single round exploded, the official said.
The rescue was complicated by the remoteness of the site, which is favoured because the harsh geography simulates conditions in Afghanistan.
Eight men were taken to a medical centre with injuries, such as penetrating trauma and fractures. One of them died, five were in a serious condition, one was in fair condition and another was discharged, said a hospital spokesman.
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 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, Mr 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 near 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 was not unusual and would persist until the investigation determined 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.