Story highlights In March, seven Marines died during training exercise

Marines did not follow correct procedures, investigation finds

No criminal charges are anticipated, the Marines said

Earlier this month, three Marines were relieved of duty related to the incident

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 session, the Marines said.

The mortar system itself functioned properly, the investigation determined.

The Marines were killed when a 60mm mortar round exploded "in the immediate vicinity" of a firing position, the Marines said.

Earlier this month, the Marines announced that three Marines were relieved of their duties after the incident.

On Wednesday, the Marines said they were relieved because their commanding general "lost trust and confidence in their ability to ensure proper preparation for, and conduct of, live fire training events."

No criminal charges are anticipated, the Marines said.

Those relieved included Lt. Col. Andrew McNulty, the officer in charge of the killed Marines' infantry battalion -- 1st Battalion, 9th Marine Regiment.

Also relieved were Capt. Kelby S. Breivogel, who commanded Company A, and Chief Warrant Officer-3 Douglas H. Derring, the battalion Marine infantry weapons officer.

The Hawthorne Army Depot, about 140 miles southeast of Reno, is used for storing ammunition and weapon stocks awaiting demilitarization. The facility also provides high desert training facilities for military units.