The security alert recommends exercising heightened caution around buses and public transport in the Las Ramblas area which was targeted last year during an attack claimed by Islamic State militants that killed 14 people. 30 police officers have been patrolling around the popular boulevard and the Christmas market near Plaza Cataluna Square. Catalonia's regional interior minister Miquel Buch said they take seriously all threats, all of which are investigated. Top-selling Spanish newspaper El Pais, citing anonymous police sources, said authorities were looking for a Moroccan man with a license to drive buses.