The United States government allowed the Sinaloa drug cartel to smuggle drugs over the border in exchange for information on rival traffickers between 2000 and 2012 according to an investigation by Mexican newspaper El Universal "that includes corroborating testimony from a DEA agent and a Justice Department official" as reported by Michael Kelley for Business Insider: "El Universal, citing court documents, reports that DEA agents met with high-level Sinaloa officials more than 50 times since 2000."
Suspicions long have existed that the Sinaloa cartel -- the 800-pound guerilla in narcoloand -- was getting a pass from law enforcement which has allowed its boss Joaquin "El Chapo" Guzman to become a Forbes-listed billionaire.
Indeed, some even conjecture that the Fast and Furious gun walking operation by U.S. law enforcement was a CIA operation to arm the Sinaloa cartel in order to defeat Los Zetas as reported by Michael Walsh for the New York Post:
Under this theory, the Agency feared the los Zetas drug cartel was becoming too powerful and might even mount a coup against the Mexican government. So some 2,000 weapons costing more than $1.25 million were deliberately channeled to the rival Sinaloa cartel, which operates along the American border, to keep the Zetas in check.
Coincidentally or otherwise, in 2011 forty high-powered assault weapons from the disasterous operation were found in the home of a reputed enforcer for the Sinaloa cartel as reported by Richard A. Serrano for the Los Angeles Times: "'These Fast and Furious guns were going to Sinaloans, and they are killing everyone down there,' said one knowledgeable U.S. government source, who asked for anonymity because of the ongoing investigations."