Well, even if one hasn't read the book or watched the deleted scenes found in The GF Saga...there is the line spoken to Michael by Don Tommasino just after Sonny is killed. Don T gloomily reports to Mike that his enemies know where he is, and he must be moved immediately. Then he informed Michael of Sonny's death.

Fabrizion selling out Michael just as he is about to relocate is good indication that ONE of those enemies got to him. Further evidence that it was Barzini may be linked to Carlo's admitting later on that it was Barzini who reached out to him.

Not only that...after the meeting of the Dons, Vito utters those famous words that '...it was Barzini all along...'.

So even if you've seen GF and only GF...the evidence of Barzini being behind almost everything that happens to the Corleones is pretty overwhelming.

Fabrizio? There's no evidence whatsoever that he was jealous of Mike & Appolonia. He wanted to come to America. The one thing that IS indicated only if you watch the deleted scene is that Barzini promises him passage to and sponsorship in the U.S. if he does this job on Michael.

Funny thing in all of this is that Barzini evidently keeps his word and brings Fabrizio over...when in reality Fabrizio FAILED in the job he was carry out. He did not kill Michael and accidently caused the death of Michael's harmless wife...in effect causing Michael to be even more dangerous to Barzini and others in the future.

AppleOnYa