Dictabelt Evidence Relating To The Assassination of John F. Kennedy - House Select Committee On Assassinations

House Select Committee On Assassinations

In December 1978, the House Select Committee on Assassinations (HSCA) had prepared a draft of its final report, concluding that Lee Harvey Oswald had acted alone as the assassin. However, after evidence from the Dictabelt recording was made available, the HSCA quickly reversed its conclusion and declared that a second gunman had fired the third of four shots heard. G. Robert Blakey, chief counsel of the HSCA, later said, "If the acoustics come out that we made a mistake somewhere, I think that would end it." Despite serious criticism of the scientific evidence and the HSCA's conclusions, speculation regarding the Dictabelt and the possibility of a second gunman persisted.

Investigators compared "impulse patterns" (suspected gunshots and associated echos) on the Dictabelt to 1978 test recordings of Carcano rifles fired in Dealey Plaza from the sixth floor of the Texas School Book Depository and from a stockade fence on the grassy knoll forward and to the right of the location of the presidential limousine. On this basis, the acoustics firm of Bolt, Beranek and Newman concluded that impulse patterns 1, 2, and 4 were shots fired from the Depository, and that there was a 50% chance that impulse pattern 3 was a shot from the grassy knoll. Acoustics analysts Mark Weiss and Ernest Aschkenasy, of Queens College, reviewed the BBN data and concluded that "with the probability of 95% or better, there was indeed a shot fired from the grassy knoll."

Dr. James E. Barger, of Bolt, Beranek and Newman, testified to the HSCA that his statistical analysis of the impulse patterns captured on the Dallas police recordings showed that the motorcycle with the open microphone was approximately "120 to 138 feet" behind the presidential limousine at the time of the first shot. When the HSCA asked Weiss about the location of the motorcycle with the open microphone—"Would you consider that to be an essential ingredient in the ultimate conclusion of your analysis?"—Weiss answered, "It is an essential component of it, because, if you do not put the motorcycle in the place that it is —the initial point of where it was receiving the —, and if you do not move it at the velocity at which it is being moved on paper in this re-creation, you do not get a good, tight pattern that compares very well with the observed impulses on the police tape recording."

The HSCA, using an amateur film shot of the motorcade, concluded that the recording originated from the motorcycle of police officer H. B. McLain, who later testified before the committee that his microphone was often stuck in the open position. However, McLain did not hear the actual recording until after his testimony, and upon hearing it he adamantly denied that the recording originated from his motorcycle. He said that the other sounds on the tape did not match his movements. Sirens are not heard on the tape until more than two minutes after what is supposed to be the sound of the shooting; however, McLain accompanied the motorcade to Parkland Hospital immediately after the shooting, with sirens blaring the entire time. When the sirens are heard on the Dictabelt recording, they rise and recede in pitch (the Doppler effect) and volume, as if passing by. McLain also said that the engine sound was clearly from a three-wheeled motorcycle, not the two-wheeler that he drove: "There's no comparison to the two sounds."

Other audio discrepancies also exist. Crowd noise is not heard on the Dictabelt recording, despite the sounds generated from the many onlookers along Dallas's Main Street and in Dealey Plaza (crowd noises can be heard on at least ten channel-2 transmissions from the motorcade). Someone is heard whistling a tune about a minute after the assassination. No one actually heard gunshots on the recording.

The only evidence that HSCA had for a second shooter was the Dictabelt sound recording. Four of the twelve HSCA members dissented to the HSCA's conclusion of conspiracy based on the acoustic findings, and a fifth thought a further study of the acoustic evidence was "necessary".