In the big, booming 1980's, South Boston was ruled by Whitey Bulger. The Boston Police couldn't stop them. The State Police couldn't catch them. And the FBI didn't even try - Bulger was on staff as valued informant. Bulger used the FBI to help him beat up on enemy gangs, and the FBI got high-profile Mafia arrests.
Bulger is still on the lam, but the FBI is being sued by the families of three of Bulger's victims. More power to 'em!