That part of the story what's most unbelievable.  The police should have brought in multiple people to review each camera in the store simultaneously.  That's how you should run an investigation of this sort.  That means at least one person will see the shooter fire on camera and see the victims BEFORE any citizens in the store pull out concealed fire arms.  They would have to be idiots to not know exactly who the shooter was.  Yes, more time will be spent viewing the tapes to ensure other people were not involved.  However, there would be no delay in know exactly who the shooter was.

Now, let's assume that they only had one person working to view security footage (an unlikely scenario).  They still know exactly which camera to check first.  That's pretty obvious.  It should have also been obvious exactly who the shooter was.