This is the general "the government killed JFK" conspiracy argument:
Earl Warren and the commission and all of the staffers - some still alive - covered up the assassination.
The HSCA covered it up. The various other smaller investigations - the Clark Panel, the Rockefeller Commission, the Church Committee - also covered it up. All of the men and women involved covered it up.
The news media's investigations - the Washington Post, the New York Times, CBS, ABC, PBS - all covered it up.
Independent investigators - a Posner, a Bugliosi - all covered it up. Various historians and other scholars like Robert Caro who found no conspiracy have covered it up. I guess we can add the very liberal Henry Steel Commager to the list (he was a vehement critic of LBJ and condemned the CIA for, in his view, acting illegally and unconstitutionally).
Fifty plus years of coverups. NOT that all of the people were wrong. Maybe they were. I don't think so. But perhaps they were misled, perhaps a sort of "groupthink" affected them. They just got it wrong. Okay. But that is not the claim. The conspiracy claim is they deliberately covered up the assassination. Why would they do that? It's never explained. They just did.
Commager was right: to the conspiracist believer nothing will dissuade them of their fixation. Every subsequent investigation that shows no conspiracy is part of the conspiracy. If we created a time machine that sent people back to observe the assassination and they found no conspiracy - Oswald acted one - that too would be said to be part of the coverup.