Blame isn't necessarily a zero sum thing.

Colt is responsible for having shot and killed those people. Fourteen is old enough to understand the consequences of shooting people.

His dad seems to be guilty of not safely securing his deadly weapons, as well as likely being negligent wrt to his troubled son.

We need more details on the FBI's involvement to know if they have any culpability. Separate from culpability, though, is the question of effectiveness, since it seems like these shooters always turn out to have been on the FBI's radar.