"U.S. citizens should exercise additional vigilance in these and similar locations, in particular during the upcoming summer travel season when large crowds may be common," the May 1 alert said. The alert said that recent incidents in France, Russia, Sweden and the United Kingdom "demonstrate that the [Islamic State] and their affiliates have the ability to plan and execute terrorist attacks in Europe," and that although local governments continue counterterrorism operations, "the Department nevertheless remains concerned about the potential for future terrorist attacks. U.S. citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning." Get the full story at Travel Weekly