Fredo had be the one ... a huge clue is the scene right after Michael and Tom are talking in the boathouse, and an hysterical Deeanna (Fredo's wife) is screming that "they" were right outside her window.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
