Seven U.S. Marines were killed and at least seven wounded when a mortar exploded during a live-fire training exercise overnight at an Army munitions depot in the Nevada desert, military officials told NBC News.

There were conflicting reports about what happened. According to one account, a 60-millimeter mortar shell exploded in a tube as Marines were preparing to fire it. Another account said that the shell exploded as Marines were picking it up to load it.

The accident happened just before 10 p.m. Monday at Hawthorne Army Depot, a 230-square-mile ammunition storage and training facility just east of the California line.

The injured were taken to two hospitals. Stacy Kendall, a spokeswoman for Renown Regional Medical Center, a trauma center about 100 miles away in Reno, said the injuries included traumas and fractures.

The Marines were part of the 2nd Marine Division, a ground combat force based at Camp Lejeune, N.C.

The depot’s website says it is a training facility for the Army, Navy and Marines, including Special Operations forces preparing to deploy to the Middle East. The site says that the facility offers a “realistic simulation of the situation in Afghanistan” because of the mountainous desert terrain.

A Marines spokesman said that the dead would be identified publicly 24 hours after their next of kin were notified.

“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 Maj. Gen. Raymond C. Fox, commanding general of the 2nd Marine Expeditionary Force, which includes the 2nd Division. “We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice.”