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MEXICO CITY (AP) -- Mexican prosecutors have launched an investigation of former officials, and three ex-governors of border state of Tamaulipas say they are the among those named in the probe.

Tamaulipas has been plagued by drug cartel violence and is the home base of two warring cartels, the Gulf and Zetas drug gangs.

The Attorney General's Office has not said why the former officials are being investigated, but by necessity it would involve federal crimes.

A statement by the office Tuesday does not name the officials, but three former Tamaulipas governors say they were placed on a travel watch issued by authorities.

There have long been suspicions that state officials may have favored the once-dominant Gulf cartel.

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