The cause of the Monday night blast was a 60 mm round that detonated inside a mortar tube, Brig. Gen. James W. Lukeman, commanding general of the 2nd Marine Division, told reporters.
The Marines said in a statement Tuesday evening that all 60 mm mortar rounds and tubes used to fire them are being pulled pending an investigation.
One Marine and one sailor suffered very serious injuries, and four Marines are in serious condition, the service said in a news release. Another Marine was treated and released. One Marine suffered minor injuries, but it is unclear whether that individual was hospitalized.
"Let me say to the families of our Marines and sailors that our hearts go out to you," said the general, whose division is based at Camp Lejeune in North Carolina. "We appreciate your sacrifice, and we'll do all that we can to continue to support you."
The Marines will release the names of the deceased 24 hours after their relatives have been notified.
Lukeman wouldn't give any details about which Marine unit was involved but said it was involved into a mountain training exercise that involved live ammunition. Marines had been training at Hawthorne and the nearby Marine mountain warfare training center in Bridgeport, California, for about a month.
Senate Majority Leader Harry Reid, who represents Nevada, called it "quite a big explosion" and said the area of the blast was cordoned off as details emerged.
"We don't know exactly what happened, but it was a violent explosion, we know that," he said. "Our thoughts are with those who were injured and, of course, the families of those who lost loved ones."
Hawthorne Army Depot, about 140 miles southeast of Reno, is used for storing ammunition and weapon stocks awaiting demilitarization. The facility also provides high desert training facilities for military units.
President Barack Obama was immediately made aware of the incident, and the White House was monitoring the situation, spokesman Jay Carney said.
"We are obviously in the early stages of assessing the incident," he said. "It is a tragedy, clearly."