Osiel Cardenas Guillen (born May 18, 1967) is a former Mexican drug lord and the former leader of the Gulf Cartel (Spanish: Cártel del Golfo) and Los Zetas. Originally a mechanic in Matamoros, Tamaulipas, where he was born, he entered the Gulf Cartel by helping Juan García Abrego, the capo at the time; when García Ábrego was arrested in 1996, some infighting erupted within the cartel. Osiel Cárdenas eventually took control by killing his friend and contender Salvador Gómez, earning Cárdenas the nickname "El Mata Amigos" (The Friend-Killer).
As confrontations with rival groups heated up, Osiel Cárdenas sought and recruited over 30 deserters of the Mexican Army's elite Grupo Aeromóvil de Fuerzas Especiales (GAFE) to form part of the cartel's armed wing. Los Zetas, as they are known, served as the hired private mercenary army of the Gulf Cartel.
After a shootout with the Mexican military in 2003, Osiel was arrested and imprisoned. In 2007 he was extradited to the U.S. and in 2010 he was sentenced to 25 years in prison for money laundering, drug trafficking, homicide, and for having threatened two U.S. federal agents in 1999.
Osiel's brother, Mario Cárdenas Guillén, worked for the Gulf Cartel, as did another brother, Antonio Ezequiel Cárdenas Guillen, who was killed by Mexican Marines on November 5, 2010.