James “Whitey” Bulger led Boston’s Winter Hill Gang for decades while secretly serving as an informant for the FBI. In exchange for providing information about rival criminals, Bulger and his associates were allowed to operate with remarkable freedom. The arrangement eventually became one of the most notorious corruption scandals in FBI history. During later court proceedings, Bulger lieutenant Kevin Weeks recalled the mob boss’s cynical view of informants: if someone else was already cooperating with authorities, there was nothing left to betray—because, “you can’t rat on a rat.”