This question has puzzled us for a long time. Consider what Michael told Tom after the shooting: "Unless I'm very wrong, they're dead already...killed by someone close to us...inside...very frightening." Not only that: The dead shooter closest to the camera has his throat cut, so the killer(s) had to be part of the shooting team.

How close? Not Tom. Probably not Rocco and/or Neri. Michael doesn't totally dismiss them as suspects, but after Michael returns from Cuba, they seem to be back in his good graces. And probably not Fredo because we see him in his robe and pj's, trying to calm his hysterical wife. Michael may have been wrong in thinking the culprit was someone "close to us...inside," unless it was Johnny Ola and his men (referred to by Michael when he sent Rocco to get something to eat for them when he met with Johnny in the boathouse). Johnny was "close to Michael" for dealmaking purposes, and he and his men were physically inside the compound. That's a big stretch, and I don't necessarily believe it. I'm grasping, just like everyone else.