FFC and Puzo were engaging in dramatic license. They left us with the mystery of who killed the Tahoe shooters to underscore the dramatic precariousness of Michael's situation at that moment ("...killed by someone very close to us...very scary..."),
Brilliantly worded, TB. I never really thought about it like that. The identity of the "mystery-men" who killed the assassins is not meant to be uncovered by the viewers OR Michael. A "very scary" and non-luminous situation indeed.
However, the assassins ending up dead outside of Fredo's window has to be taken into account. FFC is telling us with this that Fredo is involved in the assassin's deaths in some form or another. There's no way to know "how" Fredo's involved in their deaths, only that he is.
