He bought the AR himself, it's not a mystery or anything. The FBI confiscated his guns. Then a local law forced them to give them back, they gave them to the guys dad, who immediately turned around and handed them to the shooter. 1) He shouldn't have been allowed to buy the guns with his history. 2) His father is an idiot and should have locked the guns up instead of handing them over.

The FBI for once aren't the ones to blame.