Speculation about the shooter and his deeds serves no purpose. The dearth of information means either that the investigation is fruitless or that there are arrests imminent and they do not want to spook their targets.
I do not doubt that the shooter was mentally ill. He was also fixated on small arms which believe it or not, full auto fire or not, kept the casualties to a minimum.
Now I am going to demonstrate just how foolish the shooter and all of you are. If I were wanting to kill and maim hundreds at that same concert, I would not have selected any firearm as my weapon. I would have plowed through the crowd control barrier in one of these, and killed hundreds, not dozens, driving through the crowd:

(EG, this truck started life as an F-250 or F-350, it would appear.)
If I thought there would be police present, versus the unarmed security at this particular concert, then I would have specificly selected this particular model, complete with run-flat tire disks and the optional small arms armor which is proof against .50-caliber machine gun rounds, meaning that nothing the local SWAT team was equipped with could stop me from killing.

Either of these trucks is a much deadlier weapon than any small arm, and you can buy them freely, and all you need is money - no background checks for mental disease or criminal offenses. Once upon a time, I was more hesitant about discussing such topics, but the terrorists have now used vehicles as weapons of mass murder on several occasions, no longer anything to keep secret.
If I wanted to kill mass numbers of civilians, then I would arrange a huge fuel/air explosion in one of several US ports, using a hijacked LNG tanker:

...this takes a working knowledge of explosives, and a degree in Physics, Mathematics, or Engineering. Then you need access to the Civil Engineering texts on explosives - which I perused at the Engineering Library at my Alma Mater one day as an undergraduate. The equations for fuel/air explosions are relatively complex, but with a PC-based math program, easily mastered. Nothing terrorists could not acquire if they wanted to. Enough said about that, as this has not been done - yet. But I would note that Turkey just banned the transit of such tankers in river ports - for good and sufficient reasons.
The point would be that gun control is foolish, it only works for law-abiding citizens, who then become unarmed victims. It does nothing whatsoever to discourage or diminish mass murders. Indeed - the presence of enforced gun controls would cause those who want to commit acts of terror to become clever and to strike unsuspected targets using improvised weaponry, and ultimately to kill more people.
So I say let the crazies have their guns. When some fool takes an AK and sprays full auto fire into a crown, screaming "Allahu Akbar!", the SWAT team - or a crowd member with a concealed carry handgun - can put a bullet in his skull.
