I long thought Oswald was certainly the only shooter. But was their a conspiracy? Maybe. Maybe he did it all on his own for his own reasons, or maybe he was a pawn. His death at the hands of Ruby days later is the thing that makes me think there was perhaps something else going on.
(To be honest, RFK's assassination by Sirhan looks more like a conspiracy because of the shooting trajectories, but we are not discussing it here. I would not be surprised if we have "a non-conspiracy"/"a conspiracy" cases, btw).
About JFK, perhaps I give too much credence brain-wise to the CIA, the FBI, or the defence ministry. J Edgar Hoover was an idosyncratically unpleasant man, of course, but he was bright. So I always think that a higher-up conspiracy has to have a very clear plan that should work. Any leader assassination creates a precedent, too. What do we see? Nothing changed after Dallas. IRL, everything that they may have not liked JFK for, LBJ delivered.
I can imagine how scary political riots were, but after all, both JFK and LBJ were pretty normal, average, predictable imperialistic presidents operating in the Cold War era, just more liberal. (There was a limit to their liberalism, though. And, we are speaking about the time of rapid economic growth; usually at such times countries and leaders can afford being more liberal.) So it is not that anything major was achieved by JFK's assassination. Am I missing something?
Hence, I tend to cross out "serious" FBI/CIA conspiracy. I also think that in terms of his personal life, JFK was the lamest of the presidential ducks. Assassination of the leader carries a huge unpredictable risk, but anyone's reputation can be tarnished and destroyed.
So we are left with solitary groups and organizations, such as mafia, or the Cuban expats, maybe KKK. But these organizations were not worthy of a major cover-up. There were constant lawsuits against the mafia bosses, just throw the culprit to the press, the rival clan will occupy the space. Same with the rest. Am i missing anything? Was any group worthy of a country-wide coverup?
The only thing that needed to be hidden is how bad the presidential security was. And, if there were lapses in FBI/CIA work, they, too. So could it be so that what we view as the major conspiracy could be the result of minor ones, like, Hoover or McCone not wanting to lose their jobs, and definitely not James Joseph Rowley? So everyone covers lapses of own organization, we see a patchwork of "whiteouts", and together view it like a huge conspiracy?
(Oswald was such a loose cannon that he could act alone or be persuaded by anyone, but that's Oswald.)