Lincoln's assassination was the result of a conspiracy. The actual conspiracy didn't work. Booth and his associates conspired to kill the president, the vice president, and the cabinet. Lincoln was the only one killed. Seward survived an attempt to stab him. The rest of the conspirators chickened out. But we know who they were. There are a couple of open ended questions. Was Mary Surratt involved? What did Dr. Mudd know? But it's hard to keep something like that quiet.

It is hard to conceal a conspiracy. The more people involved, the harder it is. If 9/11 was a conspiracy, one of the conspirators would have told the tale by now. Nobody has.