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It seems as if some here are engaging in hindsight bias. Given the two hour delay between the shootings it was reasonable for the cops to think the shooter might have left and it was reasonable for VT to act on that by closing down the dorm, contacting the RAs to take action and notifying students of the event.
What some seem to be missing is the fact that the shooter was unknown or not identified. If you close down the entire campus, you have to search for an unknown shooter, who, is carrying concealed weapons, across an entire campus of several dozen? large buildings. If you evacuate, you have the students and everybody else in the mix with the shooter. It's very damned if you do/don't situation.
To compare this to a storm that you know is coming or an escaped convict you can readily ID is not accurate.