For fuck's sake people, there is a huge difference between saying "I'll kill you!" to someone, and telling the same person "I'm going to go home, grab my gun, go over to your house, and kill you!" In the first one, you cannot determine if it is serious. In the second...there is clearly some planning going on there. It's not like the guy said "Man, school sucks. I should go shoot it up!" He said "I'm shooting up my school tomorrow". Granted, there may be some context, such as if anyone asked him "Hey, what are you doing tomorrow", and that was his joking response. But with the evidence the police have, should they just look at the threat and say, "Meh, we'll wait to see if he actually does it"?
Seriously, people complain about school shooting and say we need better laws, better security, and all that. But when the police take pre-emptive action based on evidence, they get criticized because he may not have been serious. What do you want from the justice system, precognition?