Seven U.S. Marines from a North Carolina Unit were killed and at least seven more were injured when a mortar exploded during a training exercise at the Hawthorne Army Depot in Nevada late Monday night, the Marine Corps said this morning.

Although the cause of the accident is still under investigation, according to an official statement from the 2nd Marine investigation, NBC News reported two different accounts of what happened. One report said a 60-millimeter shell exploded in a tube while Marines prepared to fire it, and according to other, the shell exploded as the Marines were picking it up to load it.

Lt. Col. Craig Short, Hawthorne’s deputy commander, told CNN that a traffic accident at the facility just before 10 p.m. PT was connected to the explosion, but went no further in his explanation.

Senate Majority Leader Harry Reid, who represents Nevada, called it "quite a big explosion" and said the area of the blast is cordoned off as details emerge, according to new updates from CNN.

The Hawthorne Army Depot is a 147,000-acre ammunition storage and high desert training facility located about 100 miles southeast of Reno, close to the California line.

"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 Marines were part of the 2nd Division, a ground combat force stationed in Lejeune, N.C., and were in Nevada for a training mission.

The wounded marines were taken to two surrounding hospitals. The identities of those killed will be released 24 hours after their families have been notified. 

The accident occured a week after a U.S. military plane crashed during a routine training flight in Washington, killing all three crew members on board. 

Seven U.S. Marines were killed last year when two helicopters collided during an overnight training exercise at the Yuma Training Range Complex, near the border of California and Arizona.

Several other accidents also occurred last year involving Marine Corps training in Southern California. Two marines were killed in September 2012 when their helicopter crashed at Camp Pendleton, near San Diego. A similar accident killed another Marine at the same base last July.

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