Federal and local officials tightened security Friday in New York and Washington as investigators probed a possible al-Qaeda plot to detonate a vehicle-borne bomb around Sunday’s 10th anniversary of the Sept. 11 terrorist attacks. After U.S. officials received word of what they called a credible but unconfirmed threat to use a car bomb on bridges or tunnels in New York or Washington, police and other law enforcement agents in the two cities stepped up checks of such sites.