Just to play Devil's Advocate, if the FBI had acted on the tip and interviewed the young man, what would have been the next step? If the boy denied any issues, what charges could have been brought? The rifle was legally purchased, the owner was an adult, and there was no diagnosis of mental disorder. No crime had yet been committed.
On the other hand, if he had admitted problems, sought help, had his weapon secured by a responsible party, we never would have heard about it. This may happen more often then we know. We will never hear of the shootings and terrorism prevented, as it's not considered very newsworthy.
This isn't a simple case of 'he said, she said' or 'cops hands are tied'.
If reports are true - that the perp made public, recorded comments of deranged nature - then that's a solid foot in the door for a locate and interview for emergency psych commitment. Even if a temporary commitment. ANY competent mental health professional that reviewed his public comments and had him sitting down in front of them would identify his issue PDQ or convince a judge he needed to be held until it was identified.
I'm sure the state of mental health apprehensions aren't up to speed in every corner of the US, but the LE Mental Health template is well known and we have a system in place. And someone actually had enough courage and initiative to follow through and activate it.
While there may not be enough beds in the state or county facilities, this guy was an obvious, overt case that required special attention and might be worth the PITA of transporting him to a state facility in some BFE corner of the state. BTDT, you just hope it's a drive from Austin to San Antonio but if it's Austin->El Paso, then you do what it takes.
Especially damning if the Jan 5 report is true with no follow-up. Even if the fibbies were pre-occupied with the usual skels and terrorists they had a sickening amount of time to even just shove it off on local LE. I wonder if local LE was indeed contacted... I both hope so and I hope not. I bet when the shooter ID was release there were several local LE heads hung low.
On one hand, every damn system we put in place and operate is subject to human error. On the other hand we have humans assuming duties for this and nuclear weapons/power plants, child abuse/custody, etc...
As for the latter part of your quote, yes a resolution does happen more often than you realize. WAY more often. But 9/10 times the emotionally disturbed person keeps repeating the cycle until they die or are permanently incarcerated for harming family, caretakers or public safety people trying to help them.