The three Marines killed in the overnight shooting at Quantico Marine Base were all staffers of the base’s Officer Candidate School, which trains promising service members to join the Marines’ commissioned ranks.

The program, used by all the military branches, is a supplement to the service academies and Reserve Officer Training Programs that funnel in the remainder of the branch officers.

The OCS Web site says its mission is to “train, screen and evaluate candidates, who must demonstrate a high level of leadership potential and commitment to success in order to earn a commission.”

The school breaks down into three components: leadership, academics and physical training.

At the beginning of training, candidates take a physical fitness test including a 3-mile run, pull-ups and crunches. The minimum requirements, according to the OCS, are 8 pull-ups, 70 crunches and a 3-mile run in 24 minutes for men; and 50 flexed arm hangs, 70 crunches and a 3-mile run in 27 minutes for women.

The OCS site says candidates complete hikes of up to 12 miles in full combat gear and train in obstacle courses and combat conditioning.

In 1997, OCS added the Crucible, a grueling 60-hour training session where sleep-deprived candidates march extremely long distances, the Washington Post has previously reported.

The academic topics covered include land navigation, maneuver tactics and general orders, as well as history, traditions, operations and organization of the Marine Corps, the OCS says.

The motto of OCS is Ductus Exemplo, which, Latin for “lead by example.”