(CNN) — UPDATE: The blast that killed seven U.S. Marines and injured eight others Monday night during a training exercise at Nevada's Hawthorne Army Depot was caused by a mortar round that detonated in a tube, according to a military official.
ORIGINAL: Seven Marines were killed as the result of an explosion during a Marine Corps training exercise at Hawthorne Army Depot in western Nevada, the Marine Corps said Tuesday.
The cause of the incident is under investigation, the Marines said.
A representative of Nevada's Regional Emergency Medical Services Authority said four medical helicopters were sent to transport patients to hospitals.
Very little was immediately known about what caused the explosion.
Lt. Col. Craig Short, Hawthorne's deputy commander, said a traffic accident at the facility around 10 p.m. was related to the blast, but he didn't explain what the connection was.
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.
"We don't know exactly what happened, but it was a violent explosion, we know that," he said. "Our thoughts are with those who were injured and of course the families of those who lost loved ones."
The Hawthorne Army Depot, about 140 miles southeast of Reno, is used for storing ammunition and weapon stocks awaiting demilitarization. The facility also provides high desert training facilities for military units.
President Obama was immediately made aware of the incident, and the White House is monitoring the situation, spokesman Jay Carney said.
"We are obviously in the early stages of assessing the incident," he said. "It is a tragedy, clearly."