Pre-crime, like in Minority Report, predicting somewhat that someone "will" commit a crime and jail him before it does. Even if he wasn't going to do it, or was just incriminated.

Regarding bomb chemicals or helper devices, most mass killing were using guns that god forbids if you complain about someone having the right to have them. And you can make bombs with common household items (i.e. cleaning products) and use i.e. pressure cookers as devices. The possibility of false possitives, or even discretionary false possitives is too big.

And still will remain the problem that you are giving even more power to the ones that do that control. No one watches the watchers.