Hillary Clinton has said that al Qaeda is behind the threat to New York City. Secretary of State Clinton said Friday that she was going public with the information so that millions of Americans stay vigilant this weekend. Earlier, Vice President Joe Biden said the terror threat on the 9/11 anniversary comes from a "credible" source, but there's still no "certitude" that an attack is really being planned. Counterterrorism officials confirmed Thursday that they are investigating the car-bomb threat to New York or Washington. Officials tell ABC that al Qaeda chief Ayman al-Zawahiri has launched a terrorist plot for the anniversary of Sept. 11. Security officials are searching for three men believed to have entered the country in August, including one U.S. citizen. Early reports said they were looking for two rental trucks, but they have since been recovered and proved to have no connection to the suspected plot. A spokesman for the Department of Homeland Security said that in a notebook that was seized after Osama bin Laden was killed, the al Qaeda leader mentioned an attack 10 years after Sept. 11.