HSCA Vol XII, pp.24 and 25. (137) Based on the statements of these witnesses, if the smoke they reported was in fact the result of gunfire, it would have originated in the area of the top of the grassy knoll. There is no way of determining what type of ammunition was used in that "gunfire" so that it can be stated conclusively whether the smoke seen by the witnesses is consistent with smoke produced by the type of ammunition used in any gunfire from the knoll. Nevertheless, a firearms expert engaged by the committee explained that irrespective of the exact type of ammunition used, it would be possible for witnesses to have seen smoke if a gun had been fired from that arena. According to the expert, both "smokeless" and smoke-producing ammunition may leave a trace of smoke that would be visible to the eye in sunlight.(371) That is because even with smokeless ammunition, when the weapon was fired, nitrocellulose bases in the powder which are impregnated with nitroglycerin may give off smoke, albeit less smoke than black or smoke-producing ammunition.(372) In addition, residue remaining in the weapon from previous firings, as well as cleaning solution which might have been used on the weapon, could cause even more smoke to be discharged in subsequent firings of the weapon.(373) (371) Telephone interview of Monty Lutz, Wisconsin Crime Laboratories, Dec. 12, 1978, House Select Committee on Assassinations staff outside contact report (JFK Doc. No. 013637). (372) Ibid. (373) Ibid.