MONTERREY, Mexico, March 14 (Reuters) - Gunmen shot and killed the police chief of Mexico's richest municipality, his wife and two others in an attack on a jewelery store in the northern city of Monterrey on Wednesday, police said.

Gunmen killed Benjamin Espinosa, the head of police for Monterrey's wealthy San Pedro Garza Garcia district, while he and his wife were buying jewelry at a shop in the city center, police said.

Another customer and a security guard died in the attack, which had the hallmarks of a drug gang killing, according to police sources who declined to be named for safety reasons.

Local media showed photographs of the four bodies on the store's blood-spattered floor, while several bullet casings were scattered on the busy street outside.

At least 20 police and ex-police officers have been killed in drug violence this year in Monterrey, Mexico's richest city, as cartels fight over smuggling routes to the United States and hit back at a federal drive against organized crime.

Monterrey, two hours from the U.S. border, is one of Latin America's leading economic centers. It has been shaken by a rise in drug executions in an eruption of cartel violence that killed 2,000 people across Mexico in 2006.

Mexican President Felipe Calderon put troops on Monterrey's streets in January, part of a nationwide campaign to fight the cartels that channel South American cocaine into the United States.

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