Seven Lejeune Marines killed in Nevada training exercise

The cause of the accident at the Hawthorne Army Depot shortly before 10 p.m. PST Monday is under investigation, officials said in a statement from the 2nd Marine Expeditionary Force at Camp Lejuene.

It was not immediately clear how the Marines were killed. Officials earlier characterized it as an explosion, without giving specifics about what happened.

The injured were taken to area hospitals for treatment and further evaluation. The identities of those killed were not released pending notification of their families and a 24-hour waiting period after that.

Renown Regional Medical Center in Reno, the area's major trauma hospital, took nine patients, including one who died, three who are in serious condition and five who are in fair condition, according to spokeswoman Stacy Kendall. All the patients are men under the age of 30, she said. Kendall described their injuries as penetrating trauma, fractures and vascular injuries.

It wasn't immediately clear how many were airlifted and how many were transported by ground, and Kendall didn't know if any patients were sent to other hospitals. A nursing supervisor at Saint Mary's Regional Medical Center said her hospital hadn't taken patients from the incident

"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," the force's commander, Maj. Gen. Raymond Fox said in a statement. "We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."

U.S. Sen. Kay Hagan also released a statement Tuesday, expressing her sympathy.

“I was so saddened to learn about the seven Marines from Camp Lejeune who were killed last night in Nevada. Our Marines, and all of our service members, demonstrate their bravery everyday as they serve our country. My thoughts and prayers are with the families of the Marines who were killed and those who were injured, and I will continue to monitor the investigation so we can find out what happened and take appropriate steps," she said.

The Hawthorne Army Depot stores and disposes of ammunition. The facility is made up of hundreds of buildings spread over more than 230 square miles.

Hawthorne has held an important place in American military history since World War II when it became the staging area for ammunition, bombs and rockets for the war. The Nevada Division of Environmental Protection says that the depot employed more than 5,500 people at its peak. Nevada was chosen for the location because of its remoteness in the wake of a devastating explosion at the government's main depot in New Jersey in the 1920s.

It opened in September 1930 as the Naval Ammunition Depot Hawthorne and was re-designated Hawthorne Army Ammunition Plant in 1977 when it moved under the control of the Army, according to its website. In 1994, the site ended its production mission and became Hawthorne Army Depot. The site currently serves several purposes for the military, including storing ammunition and explosives and providing what the military calls an ideal training facility for special forces preparing for deployments to similar desert terrain in places like Afghanistan.