A new book raises doubts about a 1979 congressional probe that concluded U.S. 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 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. Sabato says new technologically advanced audio research conducted for his book proves the Dallas police motorcycle was not travelling as part of the presidential motorcade at the time the shots were fired.

JFK Shadow On Dallas

President John F. Kennedy and his wife, Jacqueline Kennedy, arrive at Love Field airport in Dallas, Texas on Nov. 22, 1963 - the day Kennedy was killed. (Associated Press)

The motorcycle, driven by officer Willie Price, was about 3.2 km (two miles) away at the time of the shooting, Sabato writes.

In addition, he says, 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.

Kennedy, the 35th president of the United States, was killed on Nov. 22, 1963 in Dallas Texas. The prime suspect in the shooting, Lee Harvey Oswald, was arrested on the day of the shooting but was killed two days later by Jack Ruby, before a trial could be held.