A third man has been detained in a suspected plot to detonate explosives on the city's subway system, a government official said today, as police officers searched passengers' bags on subways, buses and ferries. Authorities are holding three Al Qaeda operatives in connection with the suspected plot, although no details were available on who they are, where they were detained or what agency captured them, said the government official, who spoke on the condition of anonymity because of the sensitivity of the information. Authorities on Thursday said two men had been taken into custody. A third was captured today, the official said. The official said the informer who provided information about the men had undergone explosives training with them at an Al Qaeda camp in Afghanistan. Security in and around the city's transit network was sharply increased Thursday after city officials announced that they had been notified by federal authorities in Washington of a terrorist threat that for the first time specified the city's transit system.