There's a theory that Sam's inner circle started getting killed off before they went after Sam. The big basis for this is that Richard Cain, Giancana's courier, was killed a year before. It's something to look in to.
Cain's wikipedia page makes a references of his involvement into the investigation of Tony Accardo in 1960 and having "extensive knowledge of Outfit operations" which led to Accardo's conviction, undoubtedly given to him by Giancana