Sadly there is no answer and incidents like this will carry on happening. The gun lobby is too powerful and if they can't do anything after Sandy Hook, they won't do anything after this. You can guarantee there will be calls from some for current gun rules to be relaxed further so that concert goers could have fired back.
That NRA logic is what led to tragedies like these in the first place. Makes one wonder when the penny will finally drop.
Stricter control is the only possible remedy.