For five decades, the number of shots fired at the presidential motorcade on Nov. 22, 1963, has remained one of the greatest mysteries of John F. Kennedy’s assassination, because it made the difference between the one-shooter theory and a conspiracy. On Tuesday, Larry Sabato, head of the Center for Politics at the University of Virginia, unveiled a comprehensive study of police radio transmissions from the day of the assassination, debunking the theory of the fourth shot, hence supporting the official account that Lee Harvey Oswald alone had killed the president.