One figure who I have long felt deserves further investigation is Richard S. Cain. A good place to start is this document
Richard Cain had ties to both the Mafia and the CIA and, as this document shows, to the Fair Play for Cuba Committee, with which Oswald was reportedly associated.His brother Michael Cain is working on a book about him, which hopefully will be released soon.
This article Raising Cain is a good read. Although Cain was apparently not in Dallas on 11/22/63 his daughter believes he was involved":
“I knew my father was involved in it, or had some knowledge of it at some level,” Karla Cain is convinced. “He always talked about the murder of the President like this was just some guy he had whacked from the North Side of Chicago.”