posted by gut
I don't know, I guess it depends on which Kennedy conspiracy you're referring to. If it was as simple as the mob wanting to take him out then it's not that big of a circle. And the mob is better than most at keeping secrets.
Speaking of the mob, I guess the only conspiracy theory I really believe is that they killed Hoffa. But I'm not sure that qualifies as a conspiracy theory.
Most of the big mafia efforts over the past century have come out over and over again from people talking. Plus, unlike The Godfather, mob guys aren't very sophisticated in general. I don't think they'd have been able to keep it quiet this long - not to mention either hose or intimidate the Warren Commission in the process.