The fatal blast occurred shortly before 10 p.m. Monday during mountain warfare training in the state's high desert. The Pentagon has halted the use of the weapon pending the outcome of the investigation.
The victims were part of the 2nd Marine Expeditionary Force, based at Camp Lejeune, N.C., but were training at the Nevada facility.
The exact details of the incident were unknown. An official who spoke on condition of anonymity said it wasn't clear whether the mortar exploded prematurely inside its firing tube or whether more than a single round exploded.
The 60mm 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 rescue was complicated by the remoteness of the site since it's about a 41-minutes long helicopter ride to the nearest hospital in Reno.
Eight Marines were transported to Renown Regional Medical Center. One patient succumbed to his injuries, while five others were said to be in serious condition, one in fair condition. An eight patient was discharged, according to spokesman Mark Earnest. Hospital officials described their injuries as penetrating trauma, fractures, and vascular injuries.
All of the patients were identified as men under the age of 30. The identities of those killed were being held pending notification of kin.
Capt. Binford R. Strickland, a spokesman at Camp Lejeune, said in an email late Tuesday that he could only confirm that seven were killed and eight were injured.
The Hawthorne Army Depot, located 140 miles southeast of Reno, stores and disposes of ammunition, and provides long-term storage for industrial plant equipment. The location also provides high desert training facilities for special forces preparing for deployments in desert terrains.
"We send our prayers and condolences to the families of Marines involved in this tragic incident. We remain focused on ensuring that they are supported through this difficult time," said the force's commander, Maj. Gen. Raymond C. Fox. "We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."
The Associated Press contributed to this report.