From reply 234 above:
"So who was the shooter talking to in the hours, days, and weeks ahead of the event? Who was he meeting with? Did anyone suggest or nudge or urge him to go to the Trump rally in Butler? Did anyone suggest or point out to him the building he eventually used? Was he told at any time to not worry about security?"This reply is going to go against the grain of just about all the other responses in this thread.
Looking at this Thomas Crooks fellow...

He kinda reminds me of another shooter, almost the same age (although this photo is from a few years earlier):

I remember several years ago, after the Las Vegas Hotel shooter, how extensive investigations couldn't seem to come up with much on the guy.
I also remember posting in this forum at the time that they probably would not find out much, because (for the simple reason) there really wasn't much TO "find out" about him.
In other words, he kept his reasons to himself as he planned and executed his acts. And then he died with them.
So it was with Adam Lanza and the school shooting not far from me in Sandy Hook. Aside from a very few aspects of his personality that came out afterwards, he remains a cypher... a mystery.
So I'll predict (shakily) that it could turn out roughly the same with Crooks.
He pretty much kept to himself in general (no social media, etc.).
He probably plotted the attack on Mr. Trump on his own, and played it "by ear" as he went along. Hence, the confrontations with security at the site... but like one of those Roomba automatic vacuums, each time he "hit a bump" during such encounters, he just worked around them and tried to go a slightly different way.
And... because of the security lapses and general ineptitude of those who should have known better, Crooks kept trying and overcoming those obstacles to finally get up on the roof with his rifle. And take the shot.
So... the fact that they could find nothing of significance on his phone, and perhaps on his computer as well, seems to indicate that he may have taken his motivations to his death with him. We may never know that much more about him.
Sometimes, when nothing seems to "be there"...
... it's because nothing really IS there.