March 19, 2013 -- Updated 1555 GMT (2355 HKT) NEW: Mortar round detonated inside tube, military official says Eight others were injured. The cause of the blast was a 60 mm round that detonated inside a mortar tube, injuring the mortar crew and supporting personnel, a military official told CNN. A representative of Nevada's Regional Emergency Medical Services Authority said four medical helicopters were sent to transport patients to hospitals. Earlier, 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 was cordoned off as details emerged. "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. KOLO: Seven Marines dead in Hawthorne explosion 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. CNN's Tina Burnside contributed to this report.