The grassy knoll second-shooter claim is the most prominent sub-theory of the JFK assassination conspiracy. It holds that a second gunman fired from the wooden fence atop the grassy knoll north of Elm Street in Dealey Plaza on 22 November 1963. Evidence for the claim rests primarily on acoustic analysis of a Dictabelt police-radio recording (the basis for the HSCA 1979 conclusion of probable conspiracy), some witness testimony, and a perceived head-movement in the Zapruder film. The acoustic evidence was subsequently rejected by a National Research Council panel and the FBI. No physical evidence — no second bullet, no shell casings, no blood-spatter consistent with a frontal shot — has been produced. The Warren Commission found no credible second-shooter evidence; the HSCA concluded probable conspiracy based on the acoustic evidence; the NRC and FBI rejected that acoustic evidence. The claim is unsubstantiated but not fully resolvable.