In 1979, the House Select Committee on Assassinations released its final report. It contained a shocking finding: "Scientific acoustical evidence establishes a high probability that two gunmen fired at President John F. Kennedy."
This finding was entirely based on a sound recording — overlooked for almost 15 years — that was made at the time of the assassination. The sounds on the recording were picked up by a motorcycle policeman who had his microphone stuck in the "on" position. Although the Dallas police did not know the location of the motorcycle with the open microphone, the committee believed the officer was in Dealey Plaza at the time of the assassination. The recording is noisy with static.
But scientists hired by the committee said they could identify four gunshots on the recording — three from the Texas School Book Depository and one from the grassy knoll. Since two shooting locations means two gunmen, the committee said, President Kennedy was "probably assassinated as a result of a conspiracy."
In 1982, the National Academy of Sciences assembled a Committee on Ballistic Acoustics to re-examine the acoustic evidence. This group found that "reliable acoustic data do not support a conclusion that there was a second gunman." Richard Garwin of the Thomas J. Watson Research Center at IBM Corp., who was a member of the committee, explains how it came to its conclusion.
ABCNEWS: How was the Dictabelt recording made on Nov. 22, 1963?
Garwin: In the radio room of the Dallas Police Department on the day of the assassination there were two recorders for radio communications. There was a Dictabelt recorder and there was a Gray Audograph recorder. Channel one was the routine channel and was being recorded on the Dictabelt. Channel two was the channel associated with the motorcade and that was on the Gray Audograph.
One of the motorcycles [in the motorcade] had a stuck open microphone and channel one was recorded continuously with the sound of the motorcycle engine and whatever other sounds were picked up. So, a good while after the assassination it was thought maybe there were sounds from the shots themselves on this continuous five-minute recording.
The House Committee on Assassinations asked Bolt Beranek and Newman — Jim Barger and his colleagues — to analyze this to see whether there were records of shots. They concluded that there was evidence of gunshots on this Dictabelt recording, evidence not only for the three known shots from the Book Depository, but of another shot from the grassy knoll. They said that there was a 50 percent probability that [a fourth shot] came from the grassy knoll.
Later, two investigators at Queen's College, Mark Weiss and Ernest Aschkenasy, reviewed this work and extended it for the grassy knoll shot. They said there was a 95 percent probability of a shot from the grassy knoll.
It was Officer H.B. McLain that the committee said was riding this motorcycle [with the open microphone]. Of course, the fact that Officer McLain denies having been there did not make much difference to them. So there is some confusion.