I don't know about the FBI, but I always wondered about some very curious circumstances re. the Colombo shooting:

--How did a nobody like Johnson get a press pass and an expensive, professional Bolex movie camera?
--How did the woman with him, who acted as a distraction for Colombo while Johnson shot him, disappear into thin air?
--Why was nobody arrested or indicted in Johnson's shooting?

The finger initially pointed at Joe Gallo because he had a beef against Columbo, was demanding $100k and other big stuff, was close to blacks in prison, etc. But it soon shifted to Gambino. Columbo was Gambino's protege, and Don Carlo disapproved of the exceptionally high profile Columbo was taking in the media through his League. Bad for business.

I think it was an inside job in the Columbo family. Columbo had formed the League for selfish reasons: he was under indictment for perjury (lying on an application for a real estate license), and his son was facing a Federal felony rap for melting down silver coins for their bullion value. Columbo posed as a victim of law enforcement prejudice against Italians, a tactic that worked in his and his son's favor. But men in his organization had to resent being dragooned into doing work for the League instead of making money on the streets. And, since fear is the great power factor in Mafia, they probably felt that Columbo's denial of the existence of Mafia was reducing their ability to intimidate victims.

But, as always, money's the big factor. Columbo raked in millions in dues and contributions from the League, and he accounted for only a little of it through scholarships, etc. The surest way to encourage disloyalty in a Mob family is to refuse to share the wealth.

I don't discount some police cooperation--they hated Columbo's criticism of law enforcement, and the success he had in getting the NYTimes, FBI, etc., to drop "Mafia" and "Cosa Nostra" from their official language. Put it together: How did Johnson, basically a no-account, get press credentials and a $1200 Bolex professional movie camera? How did it happen that the revolver that was used to kill him was never traced--and no one in the pileup on top of him was ever questioned? How did the woman who was with him as a distraction, escape? Could be the police looked the other way.