The shooting of Abraham Lincoln, just 100 years ago, was not the work of an isolated fanatic, working alone. It was only one part of an organized plot by a group of dedicated enemy sympathizers who planned to kill not only the President, but also the vice-president and the secretary of state. By thus destroying the top leadership of the federal government, they hoped to create such disorder and confusion that the South would overturn its decision to surrender, made just five days before.
Thus, President Lyndon Johnson's worry about a larger plot, immediately after President Kennedy's shooting, was well-founded in his knowledge of the historical precedent.
Lincoln's vice-president, Andrew Johnson, was spared when his assigned assassin, George Atzerodt, who had no stomach for murder, went into an "oyster bay" [sic] to have a drink or two, drank himself into a state of confusion and rode off.1
Lattimer JK. The Stabbing of Lincoln's Secretary of State on the Night the President Was Shot. JAMA. 1965;192(2):99-106. doi:10.1001/jama.1965.03080150029005