NEW YORK — Police bolstered security on New York City's subways Thursday after receiving a threat that the mass transit system could be the target of a terrorist attack in coming days. An NYPD officer checks the bag of a commuter at the 96th Street Station on Thursday. By Stan Honda, AFP "We have never had before a specific threat to our subway system," Mayor Michael Bloomberg said at a news conference. "It was more specific as to target. It was more specific as to timing."Russ Knocke, a spokesman for the Department of Homeland Security, cast doubt on the threat. He said the agency "received intelligence information regarding a specific but non-credible threat to the New York subway system in recent days. The intelligence community found it to be of doubtful credibility." Police Commissioner Ray Kelly asked the public to be vigilant and said that the police presence will be increased in and around subway stations and on ferries and buses. Police will pay particular attention to baby strollers, briefcases and other containers, Kelly said. The city's security alert level remained at orange, the second-highest. (Related video: Subway security increased)Police and the Metropolitan Transportation Authority, which runs the city's subways, began random searches of commuters' bags in July after deadly bombings of a bus and subways in London. An estimated 4.5 million passengers use New York's subways every weekday.Bloomberg and Kelly gave no details about who initiated the threat other than to say that it began overseas. They said it did not appear to be linked to the observance earlier this week of Rosh Hashana, the Jewish new year, or Ramadan, the Muslim holy month of fasting that began this week. They said no suspects had been arrested in New York.Bloomberg said he'll keep riding the subway."I'm going to take the subway going uptown tonight," he said. "I've always believed the subway is safe and efficient. ... I would have no problems with my daughters taking the subway any more than me."(Hall reported from Washington, D.C. Contributing: Toni Locy, Dave Moniz, John Diamond in Washington.)