The man, identified as Floyd Ray Roseberry, 49, surrendered hours after the threat was made.
US Capitol Police had been negotiating with the driver who had parked a pick-up truck near the Library of Congress, across the street from the Capitol building and Supreme Court.
His motives remain unknown. Police are still searching the vehicle.
Congress is currently in recess, but some staff are working in the Capitol. Several office buildings were evacuated.