(CNN) -- Human error is to blame for a mortar round explosion that killed seven U.S. Marines and injured eight other service members during a training exercise in Nevada this year, the Marines said on Wednesday. An investigation of the March 18 incident at Hawthorne Army Depot revealed that "the Marines employing one of the mortars did not follow correct procedures, resulting in the detonation of a high explosive round at the mortar position," the Marines said in a statement. "The investigation also determined that the mortar section had not conducted appropriate preparatory training leading up to" the nighttime training...