MONTERREY, Mexico, Sept 13 (Reuters) - Gunmen murdered a police chief in the central Mexican state of San Luis Potosi on Thursday as he drove to a restaurant with his family, the latest killing in a spike in drug violence in Mexico.

A group of men shot dead state police chief Jaime Flores in a daylight ambush in the city of San Luis Potosi, destroying his SUV vehicle and leaving 40 bullet casings on the street, the state attorney general's office said.

"We believe drug gangs were behind this," said a spokesman who declined to be named. Flores' wife and two children survived the attack.

Following months of ferocious killings in Mexico this year, drug violence appeared to calm as President Felipe Calderon sent troops to crush warring drug cartels fighting for dominance of smuggling routes to the United States.

But violence has again flared this month, with gunmen killing two federal officers in the northern city of Monterrey and seriously injuring a public sector worker in shoot outs in the city center this week.

Mexican police found the head of a police officer kidnapped by drug hitmen in the southern state of Guerrero on Thursday, El Universal newspaper said.

More than 1,600 people have been killed in drug-related killings in Mexico so far this year and some 2,000 died in 2006.

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