There may have been warning signs and maybe not. With Columbine, those kids were living at home, technically under the supervision of parents, for what that was worth.
When you're dealing with a college situation, it's not as easy to tell what goes on with people. As a freshman, I was in a suite situation, where there were three bedrooms, one shared living room and one bathroom. Some folks won a 'lottery' and were allowed to pay extra for a single occupancy bedroom. There was one guy we rarely saw, really didn't know anything about, and never saw inside his room, as he kept odd hours. After that first year, none of us really never saw him again.
As a junior, I won one of those lotteries and was able to share a two bedroom apt. on campus with one other guy. We barely saw each other and really never knew what the other was up to.
It turns out he's a comedian now.
With commuting students, there were those that attended classes and I never had any interaction with otherwise, so that's a whole other matter.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."