I'm long out of law enforcement (15 years now, and I've never been a lawyer.... so take this for what it's worth....). From 1979 through 1995 I was in supervision up to the watch commander level and I'd want my guys to detain everyone on scene if that was at all possible. Usually violent incidents also include everyone scattering, running, and just in general doing their best to leave the area. Once you're able to contain the scene (and a shopping mall is one heck of an area to contain) , care for the wounded, make sure no other shooters are still there (very, very difficult if they're smart enough to keep a low profile), all that first responder type stuff ----- you move into the business of trying to find out what happened, who's involved (if known), and begin to assist the crew that will be doing the actual investigation... As for the crowd or anyone initially detained (maybe one in fifty might actually be a witness) you're in the business of identifying them, finding out if they need to be detained further, then clearing them out of the area to go on about their business. Anything you can do to get all the above done in a reasonable fashion will be upheld by courts in my experience. None of this involves any searches beyond a pat down for weapons if it's necessary. If you have the slightest hint that someone you're dealing with might have been involved - they're going to be handled very differently. They'll be separated from anyone else, not questioned by any officer (beyond basic I.D. or something involving the immediate safety of others), and held for investigation. With any luck the investigating crew will follow every possible procedure (all the way down to getting a warrant before doing anything with the individual's vehicle, etc.). A really professional outfit will try not to have the slightest action or procedure out of place if it's going to court eventually. If you think the general public is upset when someone gets off that did something awful you can't imagine how the cops feel about it....
I've taken the long way around to get to this point.... Major violent scenes are a pain and a half for everyone involved. Every ordinary person is being inconvenienced as they're being detained until they can be cleared, the police agency has every officer tied up (the entire next shift has probably been called in, in some cases the entire manpower of a small department and the manpower of nearby small towns as well may be needed...).
Like I said a major big deal. You can be sure that police types will move heaven and earth to clear it, if possible. That doesn't mean that ordinary shoppers may not be there for hours if necessary. Whenever I think about this sort of stuff I'm really glad I'm not in that line of work anymore.
Of course you can count on everyone with a microphone or TV camera telling anyone that will listen how terrible it all was and how everyone's "rights" were infringed upon if folks are complaining. That's the way I remember it being every time...
