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.
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.
Renown hospital emergency physician Dr. Michael Morkin, at a news conference late Tuesday afternoon, said some of the injured Marines he treated were conscious and "knew something happened but didn't know what."
He said he's "fairly confident" that one of the most seriously injured Marines wouldn't have survived had it not been for the response of a Careflight medical helicopter to the remote site near Hawthorne, 140 miles southeast of Reno.
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.
Retired Nevada state archivist Guy Rocha 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.
Hawthorne has held an important place in American military history since World War II 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.
The facility was considered safely remote, but strategically close to Navy bases in California.
Rocha said he was unaware of any other catastrophic event at the depot over the years it served as a munitions repository. The facility has downsized in recent years but survived a round of base closures nationwide in 2005.
Military officials noted that it gave Marines, Army and Navy personnel a place to train for deployment overseas.
"They train at a similar climate, elevation and terrain as Afghanistan," said Rocha, who has visited the depot many times over the years.
In the small town that calls itself "America's Patriotic Home" near the depot, a massive flag in a park across from the local war memorial waved at half-staff.
Larry Mortensen, an industrial engineer at the depot for 41 years before retiring in 1999, serves with his wife, Carole, on the board of directors of the Hawthorne Ordnance Museum. The museum displays hundreds of shells and other munitions, battery guns and weapons dating to World War II.
Mortensen said there had been fatal accidents at the depot in years past, but none resulting in mass casualties. He said he expected the rural town of about 3,500 residents to rally around victims' families.
"It's a military community. Everybody here supports the military," he said.
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, and Joseph Altman in Phoenix.