Mexico City - A couple and their three children have been shot dead in western Mexico, prosecutors say, marking the fifth killing of families in two weeks.Mexican criminal gangs used to avoid targeting the family members of their rivals, but in recent weeks, gunmen have mowed down fathers, mothers and children indiscriminately.Michoacan state prosecutors said in a statement that the latest killings occurred in the drug-plagued city of Arteaga, once the headquarters of a Knights Templar cartel leader. One person survived the attack by unidentified gunmen and is being treated at a hospital.In the normally less-violent southern state of Oaxaca, a man, a woman and their son were shot to death in the city of Juchitan recently. A hand-lettered sign left at the scene suggested the slayings were linked to a dispute between criminal gangs.Earlier eight adult members of a family were killed in the popular Oaxaca beach resort of Puerto Escondido. The prosecutor's office said that the victims were shark fishermen and that an early line of inquiry was whether they might have been involved in other activities while at sea.And in the northern border state of Tamaulipas, two families were gunned down in two separate attacks earlier this month.The violence in Tamaulipas, across the border from Texas, has not abated since then.A marine patrol on Friday intercepted the two vehicles they were travelling in. The suspects opened fire and tried to flee.The marines returned fire and both vehicles caught fire; five suspects died in the flames.