The French government called for calm Wednesday after the killing of a 22-year-old man by police sparked riots in the western city of Nantes, highlighting the simmering tensions between youths and security forces in deprived urban areas. Rioters set fire to cars and a medical center in the city on Tuesday night after news spread that an officer had shot dead the 22-year-old, named by local newspaper Ouest France as Bubakar, after stopping his car over an infraction.