As a teenager, he began to work for Los Tejas, a gang that ran the criminal activities in his hometown of Nuevo Laredo. Unlike the first members of
Los Zetas, he was never in the military. He was hired by the
Gulf Cartel in the late 1990s for his experience moving contraband across the border. When he joined the Gulf Cartel, Los Tejas, the local gang he once worked for, was absorbed by Los Lobos or Grupo L under the leadership of Treviño’s at the time direct boss, El Karis. Around 2005, Treviño Morales became the regional boss of Nuevo Laredo, replacing Sauceda Gamboa; he was in charge of fighting off the incursions of the
Sinaloa Cartel, which was attempting to take control of the smuggling routes in the area. The
Laredo–Nuevo Laredo area is a lucrative smuggling route for narcotics because of the
Interstate 35 highway, which serves as a strategic pathway to San Antonio, Austin, and Dallas for future drug distribution. While in power, he orchestrated a number of assassinations in American cities and in Mexico by young U.S. citizens whom he put on his payroll. Treviño Morales was good at identifying and grooming young teenagers who he believed had the potential to become professional assassins for Los Zetas. These recruits, sometimes called
Zetitas ("Little Zetas"), usually joined organized crime as young as twelve-years old to work first as smugglers and later as paid assassins. By 2006, the Gulf Cartel and Los Zetas managed to defeat the forces of the Sinaloa Cartel in Nuevo Laredo. The latter organization concentrated its efforts in northeastern Mexico, becoming dominant there. Los Zetas started to expand into other criminal activities beyond drug trafficking. Under Treviño Morales, the organization smuggled immigrants to the United States, carried out extortions and kidnappings, sold bootlegged CDs and DVDs, and intimidated and/or killed residents who failed to cooperate with them. Treviño Morales remained in charge of Los Zetas in the state of
Nuevo León and in
Piedras Negras, Coahuila, until March 2007. when he was extradited to the United States in 2007, Treviño Morales and
Heriberto Lazcano Lazcano pushed for Los Zetas' independence from the Gulf Cartel. In November 2007, the city of
Laredo, Texas, issued an arrest warrant for Treviño in connection with a 2006 double homicide in Texas. In 2008, Treviño Morales and Lazcano Lazcano, the two leaders of Los Zetas, forged an alliance with the
Beltrán Leyva Cartel. It had just gone to war with the Sinaloa Cartel, believing that
El Chapo Guzmán, their leader, had betrayed them. Treviño Morales subsequently joined them to kill the leader of the Sinaloa Cartel. In February 2008, Lazcano Lazcano sent Treviño Morales to kill rival drug traffickers and take control of the drug trafficking routes in Guatemala. Reportedly, he carried out a military-like ambush that resulted in the death of the Guatemalan drug lord Juan José León Ardón (alias
Juancho) in March. An unnamed U.S. official said that Treviño Morales may have been the man who fired the bullet that killed the drug kingpin. ==Leadership position==