Lincoln's assassination was part of a conspiracy. We know the names of all the conspirators, because they were caught. There are some open questions; was Mary Surratt involved? How much did Dr. Mudd know? But we know the names of Booth's fellow conspirators, and that they planned to kill Lincoln and his cabinet, because several of them told us what they knew.

I remember Watergate. John Dean couldn't keep it quiet. G.Gordon Liddy bragged about what he did.

The thing about conspiracies is that more than one person is involved. The more people who share a secret the more chances there are it won't stay secret.