***Ditto for the three fat f**ks who turned and ran the other away***
I think we seriously need to consider that the middle officer of the three was hit by one of the shots fired by the officer left frame. If so, thank goodness that officer that was hit was wearing a vest. Speaks poorly of the officer left frame, shooting without seeing the background AND missing an oncoming target at close range every time.
Also, in defense of those three officers turning back, they may have been covering the 'back door', the direction the assassin came from in case there were more shooters following.
There is much discussion of the officer and the dog. What may be a key to that scenario is 'what was that door there for?' Since it is just prior to the metal detectors, it may be the 'cloak room', where visitors would check their coats and any weapons or metal objects not needed inside the function. That may be why the canine officer was so casual at first. The perp appears to be wearing a heavy coat with long 'tails' when entering the room. No long gun was visible before he passed through the door.