It had to be. TB's point is well taken, however the fact of the matter is that Rocco was shot dead, and thus was identified by the FBI as one of Michael's hitters. I would suppose that Michael had some kind of means by which he could deny involvement but you would think that someone at the FBI might start sniffing a little pattern here....first all the enemies of the Corleones die on the same afternoon in New York, then a few years later Freank Pentangeli and Hyman Roth die at more or less the same time.