Based on apparent signs that Al Qaeda is looking to attack European cities, the State Department issued a warning to U.S. citizens in Europe today, telling them to be aware "of the potential for terrorists to attack public transportation systems and other tourist infrastructure."

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.

We continue to work closely with our European allies on the threat from international terrorism, including al-Qa’ida. Information is routinely shared between the U.S. and our key partners in order to disrupt terrorist plotting, identify and take action against potential operatives, and strengthen our defenses against potential threats.

However, the NY Times reports, "The U.S. travel alert urges extra caution and does not discourage Americans from visiting Europe. An American official who confirmed the warning on Saturday, who did not want to be identified speaking about internal government deliberations, said a stronger “travel warning” that might advise Americans not to visit Europe was not under consideration. European officials have been concerned about the impact on tourism and student travel from any official guidance to American travelers."

Countries around Europe have been raising their terror threat levels. The U.S. "National threat level" is yellow, or "Elevated," while NYC's threat level has been orange or "High"—as in "high risk for terrorist attack"—since 2001. For domestic and international airline travel, the threat level is orange.