A Marine shot dead a male and female colleague then killed himself Thursday night at Marine Base Quantico in northern Virginia.
All three were staff members at the Officer Candidates School at the base about 37 miles south of Washington, authorities said.
A personal dispute seemed to be at the heart of the incident, NBC Washington TV station News4 reported.
Police responding to reports of shots fired just before 11 p.m. initially encountered one victim, dead, and the shooter, who fled into a barracks, base commander Col. David W. Maxwell told reporters in a press conference. When authorities entered the barracks - which housed staff at the school, not students - they found the shooter and the second victim.
The names of the victims will not be released until the next of kin are notified, Maxwell said.
The base was locked down while the investigation happened. A Marine Corps official said, however, that the shooting appeared to be "an isolated incident" and the base reopened Friday morning.
Pentagon press secretary George Little said in a statement Defense Secretary Chuck Hagel was saddened to learn of the shooting.
"This tragedy, as well as the tragedy in Nevada earlier this week, took the lives of Marines who volunteered to serve their nation," Little said. "His heart and his prayers are with them and their families."
The shooting is the second tragedy the Marine Corps has faced this week. Seven members of the 2nd Marine Expeditionary Force were killed Monday when a mortar shell exploded in its firing tube during an exercise at Hawthorne Army Depot in Nevada. Eight others were injured.
Maxwell seemed emotional in the press conference, apparently choking up as he discussed the events.
The Quantico base is also home to the FBI's training academy. Its Officer Candidates School is the equivalent of basic training for Marines interested in becoming officers.
According to a Marine Corps website, the mission of Officer Candidates School is to train, screen and evaluate candidates. The training includes both academics and physical training such as endurance hikes and obstacles courses. Marines become second lieutenants on graduation from the 10-week program.