A fourth suspect has been arrested and linked to Aurora airport-shuttle driver Najibullah Zazi's plot to bomb the New York subway stations at Times Square and Grand Central Terminal. Zazi, who has pleaded guilty and is cooperating with investigators, appears to be helping out operatives in Pakistan, where he and some friends from high school allegedly traveled in 2008 to seek terror training, according to The Associated Press . Officials are saying little else about the fourth person, other than the suspect is a citizen of Pakistan. The person has not yet been charged and may not ultimately be charged. Meanwhile, two other men suspected of direct roles in the plot, Adis Medunjanin and Zarein Ahmedzay, have pleaded not guilty. New York Mayor Michael Bloomberg thinks the Zazi bombing plot and the recent deadly attack in the Moscow subway system mean terrorists still aim to harm Americans. "Anybody that thinks that we are not a target, that thinks that we are being hysterical...this should go to show that that's not the case," he says (via the New York Daily News ).