I agree it would have been someone from within the family, both due to overall trust as well as trusting the timing of the execution. I like the idea of it being Pentangeli. That would make all assassins other than Neri coming from Clemenza's caporegime. Quite a lot of trust was placed in that faction considering Clemenza was considered a risk to turn after Vito's death (not to mention initial suspicion for Vito's shooting as well).