Security Tightened in New York After Vague Threats of Terrorism
Federal and local law-enforcement officials issue warning of vague and uncorroborated threats against Brooklyn Bridge and Statue of Liberty as New York City imposes security measures not seen since first months after Sept 11 terrorist attacks; some of security measures are focused on US vessels in town for Fleet Week; officials say information came during debriefing of Al Qaeda detainee; photos (M)