'Pre-crime' detector.

Would it be ineffective towards people who would excel in remaining calm in such scenario's, though?

Most likey, especially against trained agents. However the average crazy person with a gun trying to make the headlines...

Point taken. It would indeed be a great line of defense against those who didn't take the time to mentally prepare themselves.

Eh, wouldn't it only be a matter of time before they realize that they need to calm the fuck down? They would then have to create another million? or billion? dollar experiment to combat the unthought thoughts of a killer.