[Updated at 12:29 p.m. ET] 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 60 mm round that detonated in a mortar tube, according to a military official.

[Posted at 9:22 a.m. ET] Seven U.S. Marines were killed and several others were injured during a training exercise Monday night at the Hawthorne Army Depot in Nevada, the Marine Corps said Tuesday.

The cause of the incident is under investigation, the Marines said.