Heh... as soon as you said '62, I knew what you were linking. By "no precedent", I mean there's nothing like it that's happened before. The only similarity between this and Whitman back in '62 is the setting, and the nutbar factor.
Also, stalkings are useless reports. You have to get proof of it first, and even then... the overwhelming majority line from police is "until he does something, we can't do anything" (unless you're a celebrity). At best, someone may have gotten a restraining order. Killing 32 people and injuring another how many doesn't sound like a guy who'd pay any attention to a piece of paper that says to keep back x feet.
It's not like if the stalking victims had come forward, he'd be in jail right now. If anything, there might have been more personal revenge bloodshed before the mass killings, if he'd had charges filed on him.
And again, I'm sure people come forward about disorderly, depressed, or odd behavior. Hindsight is great- if you asked any of those people prior to the incident, I don't think a single one of them would have said he was nearly disturbed enough to do this. If there was real evidence, like someone saw his threatening writings, then that's separate- planing on killing people is a punishable offense, and if someone didn't report it, then we're talking about a crime going left unnoticed.
The problem is, we live in reality... where people's confession letters of hate and anger and bloodlust aren't discovered until after the deed is done, and counselors don't (and reasonably so) assume every depressed/upset college student is a homicidal maniac.
"...he never made a specific threat to harm himself or others." <- You hit the nail on the head, right there.
Like I said... hindsight and ass-coverage... if you were an administrator there and a student made no threats against himself or others, but was depressed and seemingly angry, and you brushed him off- then he killed 32 people... would you REALLY tell the news people, the whole world, that you brushed him off? Or would you say your hands were tied to act because he didn't specifically threaten himself or others?
People do not rehearse their story before the story even exists, that happens only after 32 are dead and the finger-pointing starts up.