Four police officers killed in Mexico's most violent state / California couple killed in Mexican town plagued by violence were visiting for the holidays / Mexico's city of Culiacan besieged by organised crime amid violence in Sinaloa state / 12 people have been killed in coordinated attacks in violent Mexican city / Gang violence in Mexico: 19 bodies discovered in latest grisly find / Thousands remain fearful to return home after violence in Mexican town / Surfer murders shock peaceful Mexico community