A dozen of police officers stand at attention at the place where a police officer was killed Thursday on the Champs Elysees boulevard, Friday, April 21, 2017 in Paris. The Champs-Elysees gunman who shot and killed a police officer just days before France&#8217;s presidential election was detained in February for threatening police but then freed, two officials told The Associated Press on Friday. (AP Photo/Christophe Ena)