Easy syllogism
Premise 1: The Framers and Bombers and Inquisitors got a credible early warning about the latest school shooter, from a bail bondsman who counts as part of the "law" enforcement community. FBI didn't arrest the dude.
Premise 2: The dude is obviously retarded. He doesn't have "mental health issues", he's just a retard.
Therefore: FBI was cultivating him. FBI always prefers retards for its cultivated witches, partly because retards are easy to mold and mostly because retards make the witch target groups (Christians, Southerners, Muslims) look awful.