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From what I've read so far, the VT officials thought, via the cops, that the early morning shooting was a domestic dispute and the shooter had left the campus.
Also, when you consider that the school has 25,000 full time students and is spread over 2,600 acres, and that people were probably starting to come into the school for work and students were probably starting to head out to classes, an campus lockdown is quite a task and not an instant feat.