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7 taxi drivers shot dead outside Monterrey, Mexico

Gunmen on Tuesday shot dead seven taxi drivers on the outskirts of the industrial hub of Monterrey, which has become one of Mexico's most violent cities during a turf war between rival drug cartels.

The taxi drivers were killed as they waited at two depots next to their Volkswagen Beetles in Guadalupe, 3.7 miles from Monterrey's center.

Police officers at the scene said they did not immediately know the motive for the attack, which bore the signs of a gangland killing.

Gangsters in Mexico have targeted some taxi drivers over extortion payments, while other taxi drivers have been alleged to work as spies for drug cartels.

Monterrey, which is home to several of Mexico's biggest companies, has seen a surge in murders this year. Nuevo Leon state, which includes Monterrey, registered a total of 516 homicides in the first quarter of this year, a rise of 31 percent compared to the same period in 2011, according to the governor's office.

In total, more than 50,000 people have been killed since President Felipe Calderon took office in 2006 and began an offensive against drug cartels.

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