A forthcoming book raises serious doubts about a key piece of evidence that led a House committee to conclude in 1979 that President John F. Kennedy was likely killed as part of a conspiracy. University of Virginia political scientist Larry Sabato commissioned a scientific analysis of the Dallas Police Department’s Dictabelt recording of scanner traffic from Nov. 22, 1963, as part of his research for “The Kennedy Half Century: The Presidency, Assassination, and Lasting Legacy of John F. Kennedy.”