The Fifth Assassin by Brad Meltzer

From John Wilkes Booth to Lee Harvey Oswald, there have been more than two dozen assassination attempts on the President of the United States.
Four have been successful.
But now, Beecher White, the hero of The Inner Circle, discovers a killer in Washington, D. C., who's meticulously re-creating the crimes of  these four men. Historians have branded them as four lone wolves. But what if they were wrong? (from the back cover)
We have the book in hardcover and audiobook.
Brad Meltzer has a website: http://bradmeltzer.com/