Via State Department:
The State Department alerts U.S. citizens to the potential for terrorist attacks in Europe. Current information suggests that al-Qa’ida and affiliated organizations continue to plan terrorist attacks. European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions.
Terrorists may elect to use a variety of means and weapons and target both official and private interests. U.S. citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure. Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services. U.S. citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling.