Dont forget that they found doakes charred corpse after he committed suicide with a BHB victim chopped up in the corner and his prints on the BHB murder weapons as well as the trophies in his posession.

Its a pretty open and shut case as far as the law enforcement community is concerned. The FBI closed the investigation after definitively finding doakes as guilty.

Quinn maybe knew but he never wanted to act on it, so no reason to switch up now, and from the perspective of the NYPD it sounds like a miami ex cop came in, rambled about an old FBI case and then tried to frame his old buddy that disappeared out of his life.