WASHINGTON (REUTERS) - A new book raises doubts about a 1979 congressional probe that concluded President John F. Kennedy's assassination 50 years ago was the result of an undefined conspiracy.

The book, The Kennedy Half Century by prominent political science professor Larry Sabato of the University of Virginia, questions the conclusion by the House of Representatives Select Committee on Assassinations that a shot was fired at Mr Kennedy from the so-called grassy knoll on Nov 22, 1963, in Dallas.

Lee Harvey Oswald fired at the Kennedy motorcade from the sixth floor of the Texas School Book Depository overlooking Dealey Plaza. The House conclusion of an undefined conspiracy - with an additional shot from the grassy knoll - was drawn from an acoustical analysis of a Dictabelt recording device that was on a police officer's motorcycle.

The congressional investigators believed the motorcycle was in the Kennedy motorcade and that its Dictabelt recorded four shots. Prof Sabato said new technologically advanced audio research conducted for his book proved the Dallas police motorcycle was not travelling as part of the presidential motorcade at the time the shots were fired.

The motorcycle, driven by officer Willie Price, was about 3.2km away at the time of the shooting, Prof Sabato wrote. In addition, he said, acoustic experts found the sound "impulses" picked up by the Dictabelt that were initially believed to be gunshots could have simply been motorcycle engine noise.