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 that in response, the police officers complained to their zeta leader, whom the prosecutor could only identified as "El Caballo" 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.
In addition, Garza and Garza noted that three police officers are being sought and are at large. They are part of the group that actually carried out the abduction and execution of the mayor.
These officers were identified as Gilberto Barbosa García, shift coordinator of Police in Santiago, Juan Antonio Espinoza Gallegos and Dolores Alejandro Puente alias "El Lolo.
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.
Also Tuesday, Garza y Garza told reporters that an attack in Monterrey on guards from the FEMSA bottling company was a case of mistaken identity.
The U.S. consulate in Monterrey said in a statement Monday that the attack, which occurred outside a private school attended by many Americans, may have been an attempted kidnapping. The consulate said it appeared no U.S. families were targeted but that it was temporarily pulling diplomats' children out the school as a precaution.
Garza y Garza said the guards were attacked by Zetas gunmen who mistook them as members of the rival cartel. Two FEMSA security guards were killed, three were wounded and four were taken hostage and later released unharmed.
The four kidnapped guards told police their captors apologized before releasing them, Garza y Garza said.
FEMSA has said the guards were on standard patrols in the area when the gunmen attacked. The company has said the shooting did not appear related to any attempt to kidnap a relative of one of its executives.
Companies based in Monterrey, a business hub that is Mexico's most prosperous city, have tried to protect areas where their employees work, live or go to school amid a rising tide of drug-fueled violence.
Meanwhile, the U.S. consulate in the western city of Guadalajara said in a statement that it had suspended all travel by U.S. government personnel and their family members to the town of Yahualica because of recent gunbattles between rival drug gangs.
Mexico has seen unprecedented gang violence since President Felipe Calderon stepped up the fight against drug trafficking when he took office in December 2006, deploying thousands of troops and federal police to cartel strongholds.
Since then, more than 28,000 people have been killed in violence tied to Mexico's drug war.
The dismembered bodies of two men were hung from a bridge Tuesday on a highway leading to Acapulco, the second such discovery in three days in a region where two drug lords are fighting for control of their divided cartel.
The men were hung by their feet at the entrance of Chilpancingo, the city nearest to Acapulco along the highway connecting the Pacific coast resort to Mexico's capital, according to police in the state of Guerrero, where Acapulco is located.