A U.S. Marine guard was killed at the main gate of Camp Lejeune in North Carolina after another Marine sentry discharged an M4 rifle, the base said in a statement.
The base did not specify whether the fatal shooting was intentional.
Marine Capt. Joshua Smith said the incident was under investigation by the Naval Criminal Investigative Service — but it was not considered an act of terrorism or an active shooter event.
Base officials did not immediately identify the Marine who was killed, pending notification of next of kin. Authorities said he died of a single wound to the chest after efforts to revive him.
The alleged shooter is in custody, a U.S. Marine official told NBC News. Marine officials are not ruling out an accidental discharge.
Base spokesman Nat Fahy said the shooting took place around 5:30 p.m. Tuesday.
The camp, located outside Jacksonville, N.C., was founded in 1941. It is home to several Marine commands and one U.S. Navy command.
Courtney Kube of NBC News, The Associated Press and Reuters contributed to this report.