"Warrant officer A. Stepanov entered the guardroom after his shift in a sentry tower and fired at the chief of the guard and two other officers, injuring them, at 6:15 a.m. Moscow time (2:15 a.m. GMT) Monday and killed himself after that," the service said.
Prosecutors in Chuvashia said later Tuesday the three died, one of them shortly after the shooting and the other two in the hospital.
Prosecutors said an examination of the site was continuing and witnesses were being questioned.
Stepanov had worked in the penitentiary system for 12 years and had a distinguished service record, the service said.
Investigators have suggested a nervous breakdown exacerbated by family problems might have prompted Stepanov's actions.