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