That's part of the problem - we have no idea what it may or may not have accomplished because they didn't properly do their job. And this isn't a case of "not enough resources" or some other garbage excuse.
It takes 30 minutes, tops, to get a GJ subpoena for records. It takes another 30 minutes, tops, to review said account info when it arrives. It takes anywhere from 10-20 minutes to call the FBI office or state/local law enforcement in the jurisdiction where the person lives and tell what is going on. It would take around an hour or two to locate the guy's home and sit down with him. You walk in the front door and see an arsenal, you should probably dig a little further. By all accounts, the kid was a psycho. I'm guessing one meeting is all they would need to see that this guy was a threat.