Tourists visit the Horse Guards Parade in London on Sunday. The U.S. government has warned citizens to be vigilant while traveling in Europe because of the threat of al Qaeda commando-style attacks. But the alert is not as strong as a warning, a State Department spokesman said. (Associated Press)

Tourists visit the Horse Guards Parade in London on Sunday. The U.S. government has warned citizens to be vigilant while traveling in Europe because of the threat of al Qaeda commando-style attacks. But the alert is not as strong as a warning, a State Department spokesman said. (Associated Press)