I believe that the feds questioned almost every high level Outfit member and also some Mafiosi from different crime families regarding the deaths of both Giancana and Roselli and obviously nothing came out of it, and most of the guys even said that they never knew Roselli which were obviously lies. In plane words, Giancana was a guy with many secrets in his head and also with many old enemies(feds,cia agents and mobsters) which he created during his term as boss and it was only a matter of time until he made another mistake which obviously was his own demise. For example, according to some of the newest JFK files, Giancana was offered $150,000 (which is more than 1 mil in today's money) for Castro's assassination, which in fact never really occurred and I highly doubt that Giancana refused that kind of cash and swindled the agents whenever he could