At the time of the assassination of President Kennedy the Dallas police recorded sounds from an open microphone; these sounds have been previously analyzed by two research groups at the request of the House Select Committee on Assassinations. Both groups concluded with 95% probability that the recordings contained acoustic impulses which provide evidence for the existence of a shot from the grassy knoll area of Dealey Plaza. On the basis of these results and since shots definitely were fired from the Texas School Book Depository, the House Committee concluded that "scientific acoustical evidence establishes a high probability that two gunmen fired at President John F. Kennedy.
Report of the Committee on Ballistic Acoustics studied these reports and the Dallas Police recordings on which they are based. This book reviews the methodology employed in the evaluations of the recorded acoustic data and of the conclusions about the existence of a shot from the grassy knoll. According to this report, the acoustic analyses do not demonstrate that there was a grassy knoll shot, and in particular there is no acoustic basis for the claim of 95% probability of such a shot. The acoustic impulses attributed to gunshots were recorded about one minute after the President had been shot and the motorcade had been instructed to go to the hospital. Therefore, reliable acoustic data do not support a conclusion that there was a second gunman.