Some folks have argued that Rocco did it (or had it done). I have some sympathy for that point of view, because if Rocco was a traitor of some sort then it explains how easily he was thrown away by Michael at the end of the film.

As I've mentioned on here before, Coppola was working on GF2 in close proximity to his work on "The Conversation", which in turn was influenced by Michelangelo Antonioni's "Blow-Up." The latter two films are steeped in ambiguity, and I think that it could very well be that at least some of the ambiguity in GF2 is intentional, instead of arising from successive drafts of the script and edits made to the final film.

If that is so, then whole worlds of possibility open up, including the possibility that Michael was driving certain events that he first seems to be responding to. And if that is so, then there is a very clear candidate for who killed the shooters, and that is Bussetta.