It's got to be internal. Someone who had something to gain by Cali being out of the picture. I don't think it would be an up and coming mobster, this reeks of someone who feels slighted and wanted to make a bold move or statement. I think the theory that Gotti/JoJo/Nick are taking the family back from the Sicilians (Cefalu/Cali) may be the most accurate. Peter Gotti was acting boss before he went to prison and then the Sicilian faction just takes over? The wheel has turned, the Gotti boys are on top and the Gambino's are on the bottom.