WARNING: "yep," spoilers below
Been a long time since seeing II and STILL haven't seen III but I always felt it was Rocco as well. For the same reasons and when Michael sends him on the suicide mission it seems to clinch the knowledge that Michael figured it out and he's giving Rocco a noble death instead of a punk @ss one.
WARNING: "godfather part II" spoilers below
After the shooting in the midst of the chaos Michael has a short conversation with Tom Hagen. Tom brings up the possibility that it was Rocco or Al; Michaels top two guys, Rocco was head of security for Michaels compound at that point so he was already to blame. Michael mentions what his father taught him (IMO that's referring to how Michael figured out it was Tessio that betrayed him) then he orders Rocco to capture the assassins, he specifically tells him to keep them alive in a very forceful way. When Michael gets there they've been killed by Rocco's men which i think was Michaels test same as the test Vito taught him, i think it would be consistent if Michaels test failed though which confuses things. Still i think it was Rocco, makes the most sense.