Authorities across the nation have been warned to be extra vigilant at public-transportation hubs after the arrest of Najibullah Zazi, the 24-year-old arrested in the FBI's investigation into a plot to explode bombs in the United States. Investigators are concerned that Zazi, an airport shuttle driver, may have been planning to use backpack bombs made with hydrogen peroxide-based explosives to blow up New York City subways, and that others may now follow through with the attack plans. CNN reported that investigators are looking for "about a dozen" more people in connection to the case. Counterterrorism agents conducted numerous raids Tuesday in connection with Zazi, another indication that many of the facts surrounding the case have yet to be revealed. Hotels and stadiums are also on alert.