The US Department of State issued a travel alert on March 22nd 2016 covering the whole of Europe. This follows the March 22 Brussels attacks by ISIL.
The alert states that, “Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants, and transportation. U.S. citizens are advised to vigilance when in public places or using mass transportation. Be aware of immediate surroundings and avoid crowded places. Exercise particular caution during religious holidays and at large festivals or events.”