In early 2009, a lawyer believed to be representing the Sinaloa cartel named Humberto Loya-Castro allegedly approached DEA agents in an attempt to introduce them to a client of his – Vicente Zambada-Niebla, the son of El Mayo Zambada, and according to U.S. Justice Department indictments, ranked as high as Joaquin "El Chapo" Guzman in the Sinaloa cartel.
Loya-Castro allegedly indicated to the agents that Zambada-Niebla might be interested in cooperating with the authorities. DEA agents in Mexico apparently obtained permission from higher-ups in Washington, D.C. to conduct a preliminary introductory meeting with Zambada-Niebla, arranging to meet the lawyer and his client in Mexico City on March 18.
According to what appears to be a government response to a motion filed by Zambada-Niebla in a Chicago court (where he is now on trial), two DEA agents flew to Mexico City on March 17, where they met with their Mexico City-based counterparts; their superior in Mexico City at the time allegedly met with them and "expressed concern" about U.S. agents meeting with such a high-level member of a cartel. According to the document, the ranking agent ordered his subordinates to call off their attempts to meet with Zambada-Niebla unless they received further explicit authorization to do so.
DEA agents then allegedly met with Loya-Castro at a Mexico City hotel to break the news. But shortly after, Loya-Castro apparently returned to the hotel, Zambada-Niebla in tow. The DEA agents then allegedly informed the lawyer that they could not meet with Zambada-Niebla, who purportedly "indicated that he simply wished to convey personally his interest and willingness to cooperate with the U.S. government.
This all is supposed to have happened on March 17, 2009. In the wee hours of March 18, Zambada-Niebla was arrested by Mexican authorities in the Lomas de Pedregal neighborhood of Mexico City (pic of the house above, courtesy of Google maps). In February 2010, he was extradited to the United States.
NOTE: The information above was obtained from a PDF of what appears to be the government response to Zambada-Niebla's motion, which was posted on the web. I can't vouch for the veracity of the document, hence my use of "allegedly" and "apparently" above. More information as I find out more.