Capitol staff members were told to seek shelter where they were, and people outside were instructed to take cover.
LawNewz.com will have more on this story as it develops.
Update 3/28/16 3:46pm: A male suspect was shot by police and is being taken to a hospital.
Update 3/28/16 3:51pm: CNN reports that the lockdown has been lifted.
Update 3/28/16 4:24pm: ABC News reports that the suspect has been identified as Larry Dawson, of Tennessee. It is believed that he may be the same Larry Dawson who was arrested in October for assaulting an officer on Capitol Hill, when police tried to remove him from the House gallery for interrupting lawmakers.
[Image via screengrab]