The United States is evaluating whether to tell Americans to be "vigilant" when they visit Europe, a U.S. official said Saturday. The "travel alert," which could be issued by the State Department as early as Sunday, would be a step below a formal "travel warning," a designation telling U.S. citizens to avoid travel to particular locations. A warning designation would be devastating for tourism and study-abroad programs because of liability issues. The possible travel guidance, which was first reported by the Associated Press, was not prompted by any specific threat and will not single out any particular countries, but it would come after reports of new al-Qaeda-linked terrorist plots being discovered in Europe. The alert could note that terrorists often target tourist locations or public transportation. "Americans should still continue to travel to Europe, but they just should be cautious," the official said, speaking on the condition of anonymity because he was not authorized to speak publicly about the proposed alert.