Brennan wasn't the only person to see a rifle in the SE window.
Yeah, if you count after-the-fact mentions of "projection" or "pipe thing". Nobody, other than Euins claimed they saw their "projection" discharge.
But these other reports (if they were made at the time) make it even more incredible that the so-called "sniper's nest" wasn't located by the Keystone Kops for half an hour.