Watched this last night and thought it was excellent but also felt I'd seen it before. After a while it dawned on me....it's almost the same storyline as the real life Washington sniper case in the early 2000s.
Thats what I thought hau as soon as I saw his gun pointing out of the boot lock, but it was good, and think the answer somewhere is with the teacher, and his wife who was walking with a limp.