Authorities in northern Mexico say assailants sprayed a town hall with gunfire, killing at least three police officers.
The Nuevo Leon state attorney general's office says police found 200 shell casings from assault and semiautomatic rifles outside the Los Herreras municipal office, which also houses the town's police force.
Such weapons are often used by drug cartel hitmen. Prosecutors said Tuesday a vehicle found at the scene had "Z-40" and "Z" painted on its windows apparent references to the Zetas drug gang.
Authorities blame fighting between the Gulf cartel and the Zetas for a recent surge in violence in Nuevo Leon, which is close to Mexico's border with Texas. Los Herreras is about 70 miles west of McAllen, Texas.
The attack happned around midnight Monday.