We keep going back to the same point really - I still have no answer as to what specifically could have been done to prevent Monday's attack from happening, because the reality is that there isn't one.
You can't just stop an ideology like this and you can't prevent individuals from deciding to carry out horrific murders. There is no easy solution, as much as the right would constantly have us believe it would be easy to sort out. I'm not saying more can't be done though, there's a difference.