There is no mention in the movie of how he dies, everything else is speculation. I mentioned before FFC and MP were brilliant at only giving you just enough of the plot so you know what's going on, but not how it happened.

Clemenza in GF II didn't return due to a contract dispute and was written out. Duvall on the other hand wasn't being paid enough and demanded more money, when the studio reneged he dropped out and was written out likewise. In both cases both there characters were just mentioned ot have died as a slight character to viewer narration. FFC IMO left out the details to spark the imagination of the viewer.