We need to realize that high-profile events are high-profile because they are unlikely. And trying to stop an unlikely event is very difficult if not impossible. Predicting a school shooting or when someone who has or had a mental illness is going to shoot someone is a bit like predicting where lightning is going to strike the ground. There are some generic indicators but little that can act as an actual alarm bell.
http://www.huffingtonpost.com/paul-heroux/gun-debate-mental-illness_b_5546272.html
