Nuevo Leon state Attorney General Alejandro Garza y Garza said several of the seven police officers arrested in the killing of Santiago Mayor Edelmiro Cavazos admitted they worked for the Zetas as lookouts.
Garza y Garza said Cavazos had reprimanded some traffic police officers for wrongfully fining mountain bikers and applied disciplinary measures that included pay cuts.
"They say that since he scolded them ... they figured he was working for their enemies," the prosecutor said.
Garza y Garza said the officers complained to their cell leader who ordered the killing.
He said Cavazos, who was kidnapped last week from his home and found shot to death on a dirt road three days later, had no links to organized crime.
Cavazos' slaying comes amid increasing violence in the northeast of the country attributed to a dispute between the Gulf cartel and its former allies, the Zetas.