Mario Cardenas Guillen, alias El Gordo, is accused of being one of the two top leaders of the Gulf cartel of Mexico.MEXICO CITY — Mexico's U.S.-backed naval special forces have captured a man believed to be one of the two top leaders of the Gulf cartel, a drug-trafficking organization that once dominated the northeast border region but has recently engaged in devastating battles with the vicious Zeta paramilitary force, authorities said Tuesday.