Martin, this is not rocket science:
A) Norman told us what he heard (3 BOOMS, each accompanied in short order by 3 corresponding double clicks.
B) Brennan and Euins saw a shooter and a rifle in that window, being aimed downrange.
C) A number of others saw part of what most assuredly must have been a rifle, given the aforementioned ear & eyewitness reports.
Mr. McCLOY. Did you see the rifle explode? Did you see the flash of what was either the second or the third shot?
Mr. BRENNAN. No.
Mr. McCLOY. Could you see that he had discharged the rifle?
Mr. BRENNAN. No. For some reason I did not get an echo at any time. The first shot was positive and clear and the last shot was positive and dear, with no echo on my part.
Mr. McCLOY. Yes. But you saw him aim?
Mr. BRENNAN. Yes.
Mr. McCLOY. Did you see the rifle discharge, did you see the recoil or the flash?
Mr. BRENNAN. No.
Mr. McCLOY. But you heard the last shot.
Mr. BRENNAN. The report; yes, sir.
If you don't see a shot, then you don't see a "shooter".