As I commented to someone else a month or two ago, the only thing I'm sure of is that, at this point, we're never going to get an answer we can trust, if only because it's been so long, and so many of the people who knew, or might have known, something are now dead, many of them years later and from natural causes (plus of course Oswald, where "they had Oswald killed so he wouldn't talk" is entirely plausible but doesn't let us replace that pronoun anything more specific or useful).