(Reuters) - A blast at a U.S. Army munitions depot in Nevada that killed seven Marines in March was caused when a Marine improperly loaded a second mortar round into an already armed launching tube during a live-fire training exercise, a U.S. Marine Corps spokesman said on Thursday.
Seven Marines died and eight other service members were wounded in the incident at the Hawthorne Army Depot in western Nevada when a 60-millimeter mortar round detonated prematurely in its launching tube.
Lieutenant Adam Flores, spokesman for the 2nd Marine Division, said an internal military investigation found that a Marine had been feeding a second round into a mortar tube when a round already inside it was detonated, causing the half-loaded round to set off a massive explosion.
The March 18 blast was among the deadliest such training accidents on U.S. soil in recent years. In February 2012, seven Marines were killed when two helicopters collided during an exercise along the California-Arizona border.
In Nevada, the military found that the nighttime blast was the result of human error, inadequate training, and a systemic lack of supervision, Flores said.
Three U.S. Marine Corps officers were relieved of command after the incident, which occurred close to 10 p.m. PDT (0000 ET Tuesday) during an exercise about 92 miles southeast of Reno.
The officers will be reassigned and do not face any allegations of criminal wrongdoing.
All those killed were members of the 9th Marine Regiment, 2nd Marine Division, based at Camp Lejeune, North Carolina.
The victims, ages 19 to 16, were from towns scattered across the United States and included a would-be chef and former athletes, and all but one were veterans of the war in Afghanistan.
The internal investigation's findings were first reported by Marine Corps Times, which obtained a copy of the 19-page report and hundreds of pages of accompanying interview transcripts through a Freedom of Information Act request.
(Reporting by Eric M. Johnson in Seattle and Jonathan Kaminsky in Olympia, Wash.; Editing by Cynthia Johnston and Gunna Dickson)