WASHINGTON -- The U. S. Capitol was put on lockdown Thursday after a car chase that started at the White House ended in shots fired on Capitol Hill.Sen. Jeff Merkley, D-Ore., said he was crossing the Capitol grounds with Sens. Sherrod Brown, D-Ohio, and Bob Casey, D-Penn., headed toward Dirksen Senate Office Building when he saw multiple police cars racing up Constitution Avenue about 30 yards away.