Early word was that it had happened in or near a Senate office building, and several Capitol entrances were on lock down. D.C.'s ABC affiliate reports the most detail I've seen thus far, and puts the shooting a block or two from Dirksen Senate office building:
A U.S. Capitol Police officer shot a person at New Jersey Avenue NW and C Street NW, blocks from the Capitol, according to law enforcement sources. It happened just after 5:15 p.m. D.C. Fire and EMS spokesman Pete Piringer said the wounded person was a civilian, who had been shot by a U.S. law enforcement agency. Sources say it was U.S. Capitol Police. The wounded person was taken to the hospital for treatment. The person's condition was not immediately available.
Fox News is reporting that the incident started with Capitol Police chasing a Mercedes from Union Station. Update: Marc Ambinder reports that the shooting victim was a robbery suspect, and most of the streets near the Capitol have been reopened. Update: Politico reports that the victim, who died in the shooting, fled a traffic stop and pulled a gun on police when they gave chase.