Here is something that could have been a possible play in this:

The school shooter apparently killed his mother as she slept and had done a fair amount of "prepping" for his attack, including scrubbing evidence of what may have motivated or influenced him to do this.

http://news.yahoo.com/blogs/lookout...anning-control-former-fbi-180620270.html

What makes this different from other "pissed off guy" mass shootings is that they usually leave some sorts of cryptic warnings beforehand, and then some sort of note or log to be discovered afterward. Usually in the case of someone who was bullied, some sort of log/blog about who they are targeting and why. In this case, it's like someone was in communication with him for a while, activated him, and then made sure that once he was triggered, he would wipe some evidence of the stream of communication before the thing went down.

Given the "theory" that the mother provided the weapons, it is extremely highly unusual that he would have killed her as she slept. If he hated her, then it is likely he would have killed her in a confrontational manner. My guess, if the kid was programmed to kill, the mother would have known who the programmer was, and one of those people picked up near the scene was quite likely the person. The mother got wiped because of the information she knew.