A state police investigator says officials found the bodies in three towns of the Tierra Caliente, or the Hot Country. The region got its name for its steamy weather but also for its gang activity.
The officer spoke on condition of anonymity because he's not authorised to speak to the media. He says gunmen working for a cartel attacked the vigilantes as they stood guard at roadblocks.
"Self-defence" groups first appeared in February, saying they were standing up to the Knights Templar drug cartel. In March, authorities arrested some of the vigilantes and said some had ties to a rival drug gang.