Contained in all the irrational conspiracy theories surrounding the assassination of President John F. Kennedy has always been one kernel of rationality – the possibility that Lee Harvey Oswald did not act alone.

That was the conclusion of the House Assassinations Committee in 1979, which refuted the 1964 Warren Commission conclusion of a lone gunman and which itself was refuted by a National Academy of Sciences study in 1982. Now, a new study says the House committee was right: There were two shooters; one in the Texas Book Depository, one on the grassy knoll. Four shots were fired, not three.

The study in Science and Justice, a quarterly publication of Britain’s Forensic Science Society, evaluated the methods the Warren Commission and the NAS panel used to reach their findings. Led by D.B. Thomas, a noted scientist and assassination researcher, the peer-reviewed article finds that the NAS in particular misinterpreted the sounds recorded on police dictabelt machines and erred in dismissing a fourth shot as random noise.

The sounds of a president being murdered were recorded when a motorcycle patrolman inadvertently left his microphone on, flooding Dallas police headquarters with noise. Using sophisticated techniques, scientists working for the House committee filtered out the noise and isolated four ”events” within a 10-second period that they believed to be gunfire. The NAS panel, however, said the four noises were recorded a full minute after the actual assassination and could not possible have been gunfire.

Using the newest techniques for evaluating sounds and calculating echo sources, the new study finds that the NAS made several errors in comparing recordings made from the two police radio channels in use that day, errors that led to the establishment of a time discrepancy that did not exist. Those errors, and a failure to understand the nature of the somewhat primitive and unreliable dictabelt recording equipment, in turn led the NAS panel to misjudge the sources of the echoes of the gunfire and to conclude that the fourth sound occurred too late to be a rifle shot.

This, of course, does nothing to clear up the nearly 40 years of wild speculation and unfounded guesswork that have grown up around the tragedy. But by using the latest technology to describe what did not happen, it calls out for new inquiry to determine what did.