Actually, the movie DOES kind of hint that it was Luca.. there's a deleted scene (the same one where Hagen tells Vito about Woltz's little escapade with that 12-year old girl) where Vito says something like "Send Luca Brasi to me.. I think maybe we can reason with this Mr. Jack Woltz."
Wayne
"Finance is a gun. Politics is knowing when to pull the trigger." Don Lucchesi
