A Marine commander says an accident that killed seven Marines and injured seven other service members in Nevada happened when a mortar round exploded inside its firing tube during mountain training exercises.

Brigadier General Jim Lukeman spoke to reporters Tuesday afternoon from Camp Lejeune, N.C. The Marines are part of the 2nd Marine Expeditionary Force.

Lukeman says seven service members were injured, including three with serious injuries.

Marine Corps officials say the accident happened about 10 p.m. Monday at the Hawthorne Army Depot in rural Nevada about 130 miles southeast of Reno.

The identities of those killed won’t be released until 24 hours after their families are notified.