I was the police psychologist for the state police at Tech. I arrived on-scene about 45 minutes after the last shot was fired. I then worked the investigation for the next 32 days straight.
I can tell you unequivocally that Seung Hui Cho invested in quite a bit of planning. Among other things, he spent quite a bit of time at the range (National Forest Range outside of Blacksburg). He was observed there by a number of individuals practicing multiple target engagements and tactical reloads. Several of the people we interviewed commented that they thought he was an IDPA/USPSA shooter.
Survivors from Norris described him moving calmly from classroom to classroom; several described "rapid magazine changes", "he knew where his bullets were, kept reloading" and so forth.
He not only chained the doors but left a stick note on the door that there was a bomb and if anyone tried to remove the chain it would detonate.
He dumped his hard drive (never recovered) so we don't know what his sources were. But he planned and he practiced his plan for at least 3 months prior to acting.
"I'm your huckleberry."