This question has been subject to debate before. As far as I know there has been no definitive answer.

One of the early scripts for the movie suggest the Rosatto Brothers were involved in Clemenza's death (possibly killing him by overdose of drugs).

My own 2¢ - Francis Ford Coppola purposely left vague how Clemenza died for "payback" to Richard Castellano (because he didn't come to terms to appear in Part II).

Welcome to the boards!

Basically he was saying, that his death was not unintentional, that he was more or less whacked.

Given that parts of the Trilogy are based on a version of real-time events, that statement may have been a reference to a Bonanno family member that died of a heart attack. I believe that Joe Magliocco who succeeded Joe Profaci died of a heart attack also.

While much of the films were based on real life events, they aren't completely historically correct. FFC took editorial liberties where he saw fit. If Clemenza died of natural causes, the dialogue would have ended there, but since Willie Cicci added, "that was no heart attack", it leads us to believe that very possibly Clemenza death was something more then a "real" heart attack.

My first thoughts were that maybe, Clemenza had be poisoned, causing his "Heart attack" and because of that, Cicci says what he says.

But i would probably agree with SC.I think FFC had it like that to get back at Richard Castellano.But i don't think he realized that giving an ending like that to a great character like Clemenza,was sooo very annoying for a lot of fans.

Clemenza was the most loyal of Michael's allies and subordinates. If his death had been due to foul play, Michael would have known about it and whacked the perps. Frankie was in the midst of a dispute with the Rosato brothers, and was at the party to lobby Michael to take his side. As we saw later, Frankie claimed that Clemenza "hated them Rosatos even more than I do." So, I guess that Cici was using a figure of speech--"that was no heart attack"--to imply that the aggravation over the Rosato brothers hastened Clemenza's demise.

Not necessarily. For example, Mike didn't know that Pentangeli was still alive after he was strangled (despite Mike having informants in the Police Dept. - a point that always confused me).

This line always confused me as well, I just assumed Cicci was being sarcastic.

Adding in Clemenza's hearty appetite, that sounds very reasonable. Even though Clemenza always appeared to have things under control, like most men, as he got older he was less able to deal with aggravation. Even Vito admitted to drinking more wine then he used to.

That point troubled me as well. I'm guessing that Michael, attempting to keep as much "official" distance from his New York underboss as possible, delegated routine contacts with the Police Department to Clemenza and Frankie (though I bet Michael still pulled strings personally with NY political pezzanovanti). When the cops found Frankie, as Tom said, he was "half dead, scared, talking out loud about how you betrayed him." Detectives might have concluded that, to induce Frankie to keep talking, it'd be prudent to hustle him away and keep his survival secret, lest Michael or someone else try to whack him.

As counsel to a Senate committee about to investigate organized crime, Questadt had high-level contacts in the NYC police who would have told him about Frankie. Questadt would have seen a huge opportunity for Roth, to whom he belonged. I'm guessing he convinced the committee chairman to use his clout to wheedle Frankie away from the NYC police and spring him on Michael in a guaranteed perjury trap.

These are two troubling parts of GFII.

1. As for Cicci saying "that was no heart attack" there are many possibilities. At first I thought the Rosatos whacked him, but I agree with other members here who say if that had happened there would have been serious consequences. Moreover if it was a hit by the Rosatos Micahel would have known it was Roth who was behind it, and perhaps he would have been a little more mindful of security in his home. I can see Michael doing nothing and waiting for his chance to get Roth, but that doesn't seem to be the case. The second theory, that the Rosatos drove Clemenza to death is more likely, however a heart attack is a heart attack no matter what exacerbated the condition. My conclusion is that Cicci was exaggerating slightly (he WAS talking to Fredo after all) so he could help Frankie get his hearing with Michael, and perhaps get Michael to let him go after the Rosatos.

2. I have never understood Tom's telling Michael the intimate details of Pentangeli's state of mind moments after he was snatched by the New York police. I assume when he talks about "our people in New York," Tom is talking about people in the police department who were on the payroll....kind of like the guy who called Sonny to say that the Captain had signed out to Louis in the Bronx. The whole point of having people on the payroll is to have them provide you with timely information. For instance if Sonny's guy had called two days later to tell him about Louis' the whole plan to kill Sollozzo would have failed. Why wouldn't whoever knew about this contact someone immediately? Perhaps because they thought Pentangeli was the "go to" guy, and because Michael had disappeared. But Tom was "the don" and Tom was the one who eventually found out what happened. It seems to me that once the family thought Pentangeli had been killed (by the time he was in Cuba Michael knew Roth had given the order) that SOMEONE would have contacted their people in the Police dept. to find out what the hell was going on. Don't forget they also had Cicci. That someone, unfortunately was Tom, and I have to say he dropped the ball big time on this one.