That would be the case if at least one, and reportedly 4 police officers hadn't traded fire with him already. Presumably they would have a good idea what he looked like, and they certainly would have seen his carbine.
If a group of people came out of the bathroom and one of them was holding a carbine you'd have a pretty good idea who the shooter was, unless of course it was obvious that the crowd had captured the shooter and disarmed him. And in that case it would also be pretty easy to tell what was going on.
In an active shooter situation you may have to decide on very limited information who is the problem. The same thing is true in combat, particularly against irregular troops. Even ordinary civilians could figure out who the bad guy was pretty quickly, veterans or supposedly trained police should be very good at it.
No matter what they did if there was collateral damage there will be hell to pay.