Suppression of Massacre Reports: When Hugh Thompson's real-time reports of the civilian massacre reached Americal Division headquarters, Koster failed to order a proper investigation. Instead, the reports were minimized, ignored, or buried. The Peers Commission found that Koster's headquarters actively suppressed information about the massacre.

Acceptance of Falsified After-Action Reports: The official after-action reports from the My Lai operation claimed 128 enemy killed in action with only 3 weapons recovered and zero civilian casualties. Koster accepted these patently absurd figures without question, despite Thompson's contradictory reports and the obvious impossibility of the claimed kill ratio.

Assignment to West Point Superintendent During Cover-Up: Despite having presided over both the massacre and its cover-up, Koster was assigned as superintendent of West Point, the most prestigious command in Army education. He served in this role from 1968-1970, responsible for shaping the ethical development of future officers, while the massacre he helped conceal remained hidden.

Relief from West Point Command: After the My Lai massacre became public through Seymour Hersh's reporting, Koster was relieved of command as superintendent of West Point in 1970. His removal from the most visible leadership position in Army education was deeply embarrassing to the institution.

Demotion and Loss of Distinguished Service Medal: As administrative punishment, Koster was reduced in rank from major general to brigadier general and stripped of his Distinguished Service Medal. He became the highest-ranking U.S. Army officer demoted since the Civil War. Despite the severity of the underlying offense, the punishment was widely viewed as inadequate.

All Criminal Charges Dropped: Koster was charged with dereliction of duty and failure to obey lawful regulations in connection with the cover-up, but all charges were dropped without trial. The decision to drop charges against the highest-ranking officer involved exemplified the two-tiered justice system that protected senior leaders.

Peers Commission Findings: The Peers Commission, established by the Army to investigate the cover-up, found that Koster bore significant responsibility for suppressing investigation of the massacre. The commission documented how Koster's headquarters created a culture of impunity that enabled the cover-up.