Secret Service Agent George Hickey's Statement seems credible (November 23, 1963)

From: https://spartacus-educational.com/JFKhickey.htm

Just prior to the shooting the Presidential car turned left at the intersection and started down an incline toward an underpass followed by 679X. After a very short distance I heard a loud report which sounded like a firecracker. It appeared to come from the right and rear and seemed to me to be at ground level. I stood up and looked to my right and rear in an attempt to identify it. Nothing caught my attention except people shouting and cheering. A disturbance in 679X caused me to look forward toward the President's car. Perhaps 2 or 3 seconds elapsed from the time I looked to the rear and then looked at the President. He was slumped forward and to his left, and was straightening up to an almost erect sitting position as I turned and looked. At the moment he was almost sitting erect I heard two reports which I thought were shots and that appeared to me completely different in sound than the first report and were in such rapid succession that there seemed to be practically no time element between them. Possibly four or five seconds elapsed from the time of the first report and the last.