In March, seven Marines were killed during a live-fire training exercise at the Hawthorne Army Depot.
1st Lt. Oliver David, a spokesman at Camp Lejeune in North Carolina, says a Marine operating a 60mm mortar tube and ammunition "did not follow correct procedures." The investigation also showed the team hadn't done proper preparatory training prior to the accident.
Two officers and another Marine have already been removed from their command.
The Marines did not release a copy of the investigative report and did not want to provide any further details about the nature of the mistake.
The Marines killed ranged in age from 19 to 26. All were based at Camp Lejeune.
No criminal charges are expected.
The Associated Press contributed to this report.