At least four people have been killed in a shooting at the US Navy Yard in Washington, base spokesman Ed Buclatin said Monday.
Media reports said two police officers were among several people wounded.
There were conflicting reports about one or more attackers.
The shooting began around 8:20 am (1220 GMT) at the naval facility. About 3,000 people work in the complex, which lies along the Anacostia River about 2 kilometres south of the US Capitol.
Flights at Ronald Reagan National Airport resumed after being briefly halted after the shooting was reported.