We know thats the way it is now smooth operator, but doesn't mean it has to remain that way.
There was no central registry, depository, or linkage system in place to report or compile any of this. Well actually there are parts and pieces of one with many noticeable deficiencies. Anyone could report suspicions to the FBI or homeland security and leave it up to them. The gun side has rules and regulations administered by lawmakers and individual people who had some suspicions. There were enough hints that someone different in the same situation may have gone a little further into checking on him, reporting on him, and may have triggered some investigation. The signs were there as said, people were suspicious as noted by the university profs statement, he first thought of Holmes.
We evolve as a country through experience. Maybe we can't deal with this sort of thing yet but maybe we can learn from it all how to for next time. Laws can change, investgative bodies can learn, computer systems can be modified to better collect, catch and collate information. (if we know what we are looking for)
In this case there were some pretty high powered eyes on him. He was in the middle of a high level psych class, his professors supposedly some of the best in the business. His grades plumetting, from top of the heap to dropping out in a very short period of time. His courses included studies on abberrant behaviour.
The mentorship program is an explorable and demonstrable connection. Very early a mentor is noted in his first temporal illusion discussion. Will have to go back for the link but it is in a video showing the cover of a mentorship.
Just saying, this isn't just any class where this sort of thing can be brushed under the rug so easily. The thing they are trained for and are in training for to catch sits right in front of them. Mentorship is a close up right in your mind experience.