Wonkette

An “all clear” has been issued at Rayburn. People are now allowed to leave the building. Oh, and if being trapped in your office made you hungry, sorry — the cafeteria is now closing.So what caused this whole ruckus? According to CNN, the “suspected sounds of gunshots heard in a House office building were made by a mechanic using a pneumatic hammer on an elevator.”

Sources say Capitol ‘gunfire’ likely from air hammer [CNN]
Shots Fired? Everyone In This Office Is Safe. (3:17 PM update) [Jack's Blog]
Hotline: Rayburn Hostages FREE [Hotline On Call]