The several witnesses who either saw the sniper shooting (A), part of the gun being withdrawn(B), what appeared to be something that could be a gun(C), or movement after the third shot(D) are;
1) Howard L. Brennan(A)
2) Amos Lee Euins(A)
3) Robert H. Jackson(B)
4) Mrs. Earle Cabell(C)
5) James N. Crawford(D)
Since when is seeing "movement" evidence of anything?
Brennan does not belong in category A.
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.
Besides Brennan never said how he knew the guy in position for the final shot (and thus blocked by boxes) was the same guy he saw earlier sitting on the window sill. Perhaps he just assumed it. He also never explained how how could see the guy from the belt up at the time of the last shot.
Brennan embellished his story more and more every time he told it. Have you read his book?
Only Euins actually claimed to see a person firing a rifle, and the person he saw had a bald spot. He also told one reporter that it was a "colored man".