I would feel the same way about the Air Force dropping the ball in this case. However, it was the local police who dropped the ball in the school shooting. It was up to them to investigate this kid. And up to them to call in the FBI if they needed them.
The FBI had a tip that should have been followed up, and they admit they failed to follow their own protocol.
The FBI had their tip on January 5th of this year, and it was specific as to killing people and shooting up a school, while the local police visits to the shooter's home took place over several years.
It's pretty clear the FBI had probably the most recent and most specific tip, but the local police might also have failed to follow up sufficiently over time, but I haven't seen much detail on that.