A guy named Gus Farace. I don't know if he was a "gangster" or just an associate of some family, but during a meeting regarding a drug deal he supposedly decides the guy is an informant and kills him. Turns out the guy IS actually a cop (or an FBI agent, don't remember which).

The guy goes into hiding, even makes Americas Most Wanted TV show and it ends up that the Mob actually kills him to get the heat off of them for the cop being killed.