When Fredo and Pentangeli meet in Nevada in The Godfather Part II, they talk about Clemenza's death. Willi Cicci says that was no heart attack followed by Pentangeli saying that is the reason for him to visit Michael. When later he meets Mike, he goes on to complain about the Rosato Brothers stepping over their territory. Could the two be related?
Interestingly Cicci's line about it not being a heart attack does not appear in several drafts of the script.
PENTANGELI You remember Willy Cicci, don't you, Freddie? We was all together with the old man Clemenza in Brooklyn... before...uh... FREDO We were all upset about that. PENTANGELI That's what I'm here to talk to your brother about. What's with him, I got to get a letter of introduction to have a 'sitdown'?
A similarly dated version
PENTANGELI Sure, Pete Clemenza died of a heart attack, but the Rosato Brothers gave it to him. MICHAEL We were all heartbroken at the news; but that wasn't cause to start a war.
This version which does not appear in the movie at least has the implication that the stress of dealing with the brothers caused the heart attack.
In light of Michaels conversation with Pentagelli the idea that the Rosato's were directly involved in Clemeza's death does not really hold up. Frank isn't going to war with the Rosato brother because they killed Clemenza but because they are infringing in his territory. If the brothers had killed Clemenza I think it likely that THAT would swing Michael to Frankie's side and it would have been mentioned.