. >> whitaker: and tomas zeron was that voice on the mexican side. >> attencion! >> whitaker: zeron is chief of the investigative agency of the prosecutor general of the republic, mexico's version of the f.b.i. the main u.s. assistance was surveillance-- wire tapping, phone surveillance, and that sort of thing? >> tomas zeron ( translated ): i will just tell you, they help us with technology, because they're really good in technology, and that will encapsulate everything. >> whitaker: zeron, in turn, tapped the mexican marines to execute the plan. they are elite special forces, highly-trained and known for using deadly force. we watched one of their training sessions, an assault on a mock- up of a cartel hideout. and we traveled with the marines to sinaloa, a remote state on the pacific coast, and to its capital, culiacan, chapo's base of operations where his cartel still rules. it was here in culiacan, on february 12, after weeks of secret surveillance, the marines started to move on the cartel, rounding up chapo's henchmen, who started to talk. >> maltz: there w