Andrew Arconti, chief of plans and operations at the base, told AFP that the ostensible incident was a planned exercise for such emergencies, after another official had confirmed the incident to AFP.

"The reports of a shooting at Fort Meade are false. This was a planned exercise," the base said in a subsequent statement.

Initially, various base officials had confirmed to US media that there had been an "active shooter" who had been "contained."

The initial reports drew attention because Ft. Meade, in Maryland, about 25 miles (40 kilometers) north of Washington, is home to the government's largest and most secretive signals intelligence agency, the National Security Agency, as well as the US Cyber Command, which conducts offensive and defensive cyber operations.