With Sinaloa cartel boss Joaquín "El Chapo" Guzmán 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 US, prosecutors revealed in a recent indictment. Although Guzmán's trial revolved around cocaine shipments, the case against his sons exposes the inner workings of a cartel undergoing a generational shift as it worked "to manufacture the most potent fentanyl and to sell it in the United States at the lowest price," according to the indictment unsealed April 14 in Manhattan. Synthetic…