Police in New York and Washington D.C. are out in force after intelligence officials received word of a possible al Qaeda plot to set off a car bomb around the time of the 10th anniversary of the September 11 attacks. Police in both cities are searching the bags of passengers on mass transit and checking cars at the entrances of bridges and tunnels. In Washington, the police chief warned that unattended cars parked near critical buildings would be towed. Information about the plot was passed to American intelligence officers Wednesday by a source who has proved reliable in the past. But the informer's knowledge seemed second- or third-hand, and the description of the suspects was vague: They are men, one 5 feet tall and the other 5-foot-8, and the name of one might be Suliman. Government officials say the suspects may be U.S. citizens. “All this information is very, very sketchy,” said a law-enforcement official.