In addition to those killed, a Navy corpsman was considered very seriously injured, the military said in announcing the names of the dead Wednesday night. Five Marines were seriously injured and two Marines were treated for minor injuries and released.
It was not clear where the Marines were standing or what caused the blast, which Brig. Gen. Lukeman said was under investigation.
The Marines had been training at the Hawthorne Army Depot and the nearby Mountain Warfare Training Center for the past month, Lukeman said. The training was not in anticipation of an imminent deployment, he said.
The mountainous desert terrain of the 230-square-mile depot was used as a training location for special forces, since it "provides a realistic simulation of the situation in Afghanistan," according to the depot's website. The depot is also used as a storage site for ammunition awaiting demilitarization.
It takes several Marines to fire a 60mm mortar, and they must "work together to provide constant and accurate high-angle suppressive fire," according to the Marines' website.
The mortar the Marines were using is a "lightweight company mortar fired from a stationary position," Lukeman said.
ABC News' Michael S. James contributed to this report.