Throughout most of the late 1970s and early 1980s, the Mexican drug traffickers were also middlemen for the Colombian trafficking groups, and transported cocaine through the U.S.-Mexico border. Mexico, however, remained a secondary route for the Colombians, given that most of the drugs trafficked by their cartels were smuggled through the Caribbean and the Florida corridor. Félix Gallardo was the leading drug baron in Mexico and friend of Juan Ramón Matta-Ballesteros, but his operations were still limited by his counterparts in South America. In the mid-1980s, however, the U.S. government increased law enforcement surveillance and put pressure on the Medellín and Cali cartels by effectively reducing the drug trafficking operations in the Caribbean corridor. Realizing it was more profitable to hand over the operations to their Mexican counterparts, the Colombian cartels gave Félix Gallardo more control over their drug shipments. This power shift gave the Mexican organized crime groups more leverage over their Central American and South American counterparts. During the 1980s, however, the Drug Enforcement Administration (DEA) was conducting undercover groundwork in Mexico, where several of its agents worked as informants.