[Rolo]

It might have been "a lone gunman", and the quick coverup was probably not to protect some major conspiracy to kill the president, but to protect the country from being drawn into war, if a long investigation had shown that the lone gunman was connected to another country or political movement... it was convenient that Lee Harvey Oswald was killed, because it left the system with a chance to draw its own conclusions, and not be trapped in events started by "a lone gunman".