The police tape and the fourth shot

A Dallas police motorcycle microphone stuck open and transmitted on channel 1, which was recorded on a Dictabelt. The committee’s experts, first James Barger and then Mark Weiss and Ernest Aschkenasy, fired test shots in Dealey Plaza in 1978 and compared the echoes to impulses on the tape. Weiss and Aschkenasy concluded there was a probability of 95% or better that a shot was fired from the grassy knoll; Barger agreed. Their section of the report was titled “High Probability That Two Gunmen Fired.” [1]

Based on that, the committee concluded Kennedy was “probably assassinated as a result of a conspiracy.” It also found the knoll shot missed: its pathologists found no wound from the front. [2][3]

Steve Barber, a private citizen in Mansfield, Ohio, suggested that a nearly unintelligible voice laid over the supposed shots was crosstalk from channel 2. Spectrograms confirmed that the “hold everything...” message from channel 2 sits on the channel 1 Dictabelt at the same place as the impulses, and the channel 2 recording shows that message went out one minute after the assassination. [1]

The National Research Council panel, chaired by Harvard physicist Norman Ramsey, found unanimously that the acoustic analyses “do not demonstrate that there was a grassy knoll shot,” that there was “no acoustic basis for the claim of 95% probability,” and that the impulses were “recorded about one minute after the President had been shot.” It added that sirens heard on the tape did not fit a motorcycle in the motorcade and that McLain was photographed at Parkland beside Mrs. Kennedy’s car. “Reliable acoustic data do not support a conclusion that there was a second gunman.” [1]

In 2001 Donald Thomas, a government scientist, published a peer-reviewed reanalysis arguing the synchronization was wrong and the knoll shot real. In 2005 Ralph Linsker, Richard Garwin and others replied with more crosstalk evidence placing the sounds after the shooting. [2][3]