I think early reports were that people couldn't believe that one person could have killed so many people without lining them up and killing them execution style, but the more recent reports from eyewitnesses say that he walked into classrooms, sprayed them with bullets, and then left to reload ... there was no lining up to be shot, and not a lot of time to react - maybe a minute or two in each classroom.
The students that did have time to react when they saw him coming in the hallway appeared to have fought back by barricading their doors and holding them shut even when bullets came through the doors.
Today's Post article is a pretty detailed recounting of the events:
http://www.washingtonpost.com/wp-dyn...l?hpid=topnews