Investigators said all six were shot execution-style, CNN reported. The other five, four young women and a young man, were still being identified.
Jose Armando Moreno Leos, 13, was arrested in February. Despite his admission that he had participated in 10 homicides, he was released, Arturo Nahle Garcia, state's attorney in Zacatecas, said.
Under Mexican law, children under the age of 14 cannot be jailed or imprisoned. A court hearing was scheduled Thursday to try to work out a plan for the boy, but only Moreno's mother appeared, telling a judge she had been unable to control her son since he was 11.
"It's really unfortunate, but we're seeing more and more young men who drop out of school and end up selling drugs on the streets," Nahle Garcia said. "They all end up the same. They either end up in jail or the cemetery."