MEXICO CITY, May 11 (Reuters) - Suspected drug gang gunmen killed four Mexican policemen who worked as bodyguards for the family of the country's most influential state governor, a local government source said on Friday.

The officers, assigned to protect the family of State of Mexico Gov. Enrique Pena, were shot dead as they drove their sports utility vehicle in the Gulf port city of Veracruz where the governor's young children were on vacation.

Mexican President Felipe Calderon has sent thousands of troops and federal police to tackle drug cartels across Mexico, but the increased firepower has failed to contain the violence, including a recent wave of attacks on police.

Pena, from the opposition Institutional Revolutionary Party, is one of the rising stars of Mexican politics and is often mentioned as a possible presidential candidate in the next election in 2012. His state, next to Mexico City, is home to 14 million people and is an important industrial base.

The drug hitmen, shooting from at least two vehicles, may have mistaken the officers for a rival drug gang, the source in the State of Mexico government said. Cartel members in Mexico often drive powerful sports utility vehicles like the one the police were in.

"Early indications suggest there was some confusion," the source told Reuters.

Photographs posted on newspapers' Web sites showed the windshield of the officers' vehicle riddled with bullets.

The policemen were off duty and Pena's family were elsewhere in Veracruz when the shooting happened.

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