looking at the time things happened from the more informative posts here, it looks like mom left, dad had custody, kid started posting about shooting up the school, then FBI visited. kid started posting about spree killers and being afraid of sirens after that.
think it was the DoJ that had the statistics that mass shooting perpetrators had 3 common background traits- being male, domestic violence reported in the household (by/against anyone in the household), and a felony of aggrievement about a perceived hurt (real or not). Mom, crazy as she might be, leaving the house, that's enough of a hurt for a kid that age.
if the feds or local cops "monitored" every shitposting teenager they'd be doing nothing else, there's too many of those.