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I have a friend with a little knowledge of the situation (so this is a confirmed rumor, but nothing official) that says it initially looked like a boyfriend/girlfriend thing that ended tragically. Awful, but not the sort of thing that usually ends in a massacre. 20/20 hindsight says the school "should" have known, but realistically, the probability was low enough that locking down the school wouldn't have seemed necessary.
The finger-pointing is going to get bad: blame the university, NRA, whatever. When it comes down to it, the gunman is responsible, and it's almost too bad he isn't alive to be shot 29 more times.