Neri.

By the way, in the novel Michael states that he will instruct Clemenza personally. However, as in the film, if Clemenza murders Cuneo(?) in the elevator, Clemenza would have had to be brought in on the plan earlier than that day. In addition, since Rocco worked for Clemenza, I don't see how Clemenza could not have noticed what was going on.

I just think that Puzo and FFC put themselves in a little box. Obviously, it's not until the funeral that Tessio reveals himself. So, to that point since the traitor could have been Clemenza, he couldn't have been brought in on things.