“Scratchin my head why a cop doesn’t go after a shooter?...he shoulders his weapon then gives a thumbs up into the hallway?”
It looks to me like the cop (all in black with the rifle) checks the shooter out and recognizes the shooter as a cop (Capitol Police) or U.S. Secret Service. That’s what the thumbs up means - you’re on my team.
ThankQ. that seems to be the size of it.