Thank you for responding! Short answer: Misdirection: perp used the security camera to lay a false trail for LE to follow.
Oops! I forgot to mention Victim 1 in my report. HS V1 was female, a senior (whose father helped build that forgotten bomb shelter). She was killed at her locker, near the rear of the school. Video of the shooter came from a camera aimed at the (locked) exit door, closest to V1's locker.
LE assumed a camo-perp followed a logical path from V1, to the next closest victim, to the next, and so on, to the front of the school, where (s)he exited. Actually, the shooter took the tunnel to the bomb shelter, from bomb shelter into cafeteria, cafeteria to storeroom, storeroom to ceiling stash, to cafeteria, to hallway, to gym, to hallway, to cafeteria, to different hallway, to classroom, etc. Like that. Shooter kept LE from discovering any evidence at all, for several hours.
This was Decker's high school. He was a football star, a local legend who went on to play in the NFL. He was injured on the first play of his first pro game, never to return to football. After the injury, Decker went to a rehab for people with memory issues from trauma, where he had to decide a new life plan. He announced his plan to the other patients, one of whom was the sixteen year old rape victim. She felt insulted and disrespected, and threatened, by Decker's plan, although Decker did not know her history. She knew Decker's, because he was well known as a football player, and everybody had seen him injured on TV, and on Youtube. Decker = football. Football = high school + Decker = LE x 20 years. So Decker became the focus for her anger, and the target for her revenge.
It's a good book! Complicated, but not hard to follow. I'm leaving out a lot, I know. Thanks for thinking through it with me!