To Mr Truly, at that immediate time post assassination - Givens could have been the actual assassin or a co conspirator.
. . .
Givens was missing from the line up as well.
. . .
Why did Mr Truly report only Lee "missing"?
Oswald was the natural one for Mr. Truly to suspect. He saw a police officer point a gun at Oswald. He vouched for Oswald because he didn?t realize the shots came from that building, let alone from one of his employees. When he heard a rifle was found, naturally his mind would go back to Officer Baker pointing a gun at Oswald.
If Officer Baker had initially suspected Givens but Truly immediately focused on Oswald, that would be strange. But knowing the shots were fired from that building. The shooter might be one of his employees. And an officer initially suspected Oswald. Naturally his suspicious are going to be initially turned to Oswald. And enhanced when he also realizes he hasn?t seen Oswald since that encounter with Officer Baker.