Drudge is reporting "Police sealed off the U.S. Capitol Friday after reports of gunfire in a House office building across the street..."
From the Hill, Reader RM writes "shots fired in Rayburn," a building in the Capitol complex which contains offices for House members and committees.
Update: WPost says, "Shots reported fired at the Rayburn House office building, employees ordered to shelter in place, according to television news reports."
Update: CNN says shots are unconfirmed, police are investigating "sounds of gunfire" in Rayburn garage.
Update: Congressional Quarterly says the House has left for the long weekend, and has only two hearings scheduled, both in Rayburn. One is a subcommittee markup of a 2007 appropriations bill; the other, an intelligence committee hearing entitled, "The Media's Role and Responsibilities in Leaks of Classified Information."
Update: CNN reports more than one "gunshot" was heard, and they were reported by a single staffer.
Later Update: Capitol Police will be holding a presser at noon.