...or both
At the time of the murder, Brennan was just another good citizen who wanted to help the police catch the criminal..... So when he saw all of the police going to the rail yard behind the GK, he wanted to alert a cop to the fact that he'd seen a man with a hunting rifle aiming it out of an upper story window. So he dashed over to the cop and told him what he had seen....
Later after he realized that the police were only interested in framing Lee Oswald he became a bit unhinged.....a loon.
I suspect that Brennan's mind may have tricked him into thinking that what he had see just a couple of minutes before the shooting took place during the shooting....
IOW.... Brennan saw basically the same thing that Arnold Rowland saw ...except Brennan actually saw the khaki clad man aiming the rifle out of the window, as he scanned the crowd through the telescopic sight, just as hunters frequently do. Then a couple of minutes later after the shooting he imagined that he had seen the man aiming the rifle DURING the shooting. Yes that's entirely speculative.... BUT... it's feasible and there's no reasonable rebuttal for Brennan's story. The WC recognized that they had to discredit Brennan's story so they made him a star witness so they could twist his story and manipulate a man who was full of fear after seeing Lee Oswald murdered while being escorted by the very same cops who he had desperately tried to tell that Lee was NOT the man that he had seen aiming a rifle out of a TSBD window.