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 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.
This week, in the normally less-violent southern state of Oaxaca a man, a woman and their son were shot to death in the southern city of Juchitan.
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Three days 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.
In the first attack, gunmen entered a home and killed 11 members of the same family while they were sleeping, including four girls.