Guillermo Diaz was a Mexican drug trafficker who was affiliated with the Sonora Cartel. In 2015, he was arrested in a joint US-Mexican operation in Ciudad Juarez, and an attempt by the cartel to rescue him at the border failed. Diaz was interrogated by CIA officer Matt Graver and assassin Alejandro Gillick, and he was forced to reveal the existence of a smuggling tunnel at Nogales.