Now there's a phrase which we are all getting pretty familiar with.
So what's the law on this?
Identify the risk as a potential mass murder but don't actually deal with the suspect until he has killed innocents and himself?
I've got a much simpler idea.
If the suspect is known to be a threat, strike first.
Don't wait until after the inevitable atrocity and then claim, "Yeah, we knew about him."
Bit too late for that, don't you think?