If you know just a bit of how security details work, shooting, and stuff, you will conclude that the simplest and most likely answer is that, yes, it was a semi-random guy -- who got the assistance of a three letter agency. THAT is.

A random amateur simply does not get to carry a rifle and take a vantage point barely 100 yards away from a presidential target surrounded by secret service without help/assistance/someone looking the other way and making sure that nobody looks either. The probability of that would be ZERO.

Plus, to begin with, the "deranged shooter" myth has been debunked way too many times now. There are deranged people who would like to shoot other people, indeed. Taking the final step of making the decision, making the plan, organizing it... for that, more than one has been known to need a "nudge" by an "interested party", if not outright brainwashed and put up for it by them directly.