Security officers found "no indication" of an active shooter after reports from Walter Reed National Military Medical Centre outside Washington.
Naval Support Activity Bethesda tweeted that an active shooter had been reported in the basement of a building at the Maryland base.
NSAB public affairs officer Jeremy Brooks told AP by phone that "right now there is no indication that there is any active shooter, but we won't know 100 per cent until the buildings have been successfully cleared".
Brooks said "we've stopped all traffic coming in or out".