Again, thanks for all of the replies.
I guess my original question really wasn't clear. This has nothing to do with what I could do (based on training, weapon available), want to do (my instinct, like others, would be to help) or should (ethically) do. Those decisions may vary based on the situation at hand and all of the variables that you guys have mentioned.
I guess I was trying to get a LEO's perspective as to what their thoughts are of a civilian CCW holder becoming involved in such a situation. Would it make it more difficult for them (assuming minimal communication)? What would they want a civilian CCW holder to do/not do should the civilian make the choice to become involved? Does their training account for such a possibility and how do they handle? Is a civilian allowed to assist or are they assumed a threat as well until things are identified and sorted out?
Like all of you, I would want to assist and hopefully stop such a terrible incident, but I would not want to make the situation worse for the professionals responding.