Police and military patrol Culiacan, Sinaloa state, Mexico, on Jan. 6, the day after the government detained Ovidio Guzman, the son of imprisoned drug lord Joaquin 'El Chapo' Guzman.Martin Urista/The Associated Press With Sinaloa cartel boss Joaquin “El Chapo” Guzman serving a life sentence, his sons steered the family business into fentanyl, establishing a network of labs churning out massive quantities of the cheap, deadly drug that they smuggled into the U.S., prosecutors revealed in a recent indictment. Although Guzman’s trial revolved around cocaine shipments, the case against his sons exposes the inner workings of a cartel undergoing a generational...