Reportedly there were more than 20 guns found in the hotel room, which include fully automatic rifles. It is nearly impossible to believe this person is a "lone wolf." How do you go about acquiring that many firearms without some help? I am inclined to believe there is a "support group" of sort that enabled this man.
Assuming he did this, that is. I cannot rule out a possibility of him being a victim himself to a real perpetrator who staged a cover-up, considering the magnitude of violence involved here.
It absolutely is in the realm of possibility for it to be a lone "actor", as guns do have this level of ability to impart such harm.
Undoubtedly we'll see all manner of conspiracy theories (just like JFK) crop up. We've already seen right-wing nutters (and/or Russians fomenting FUD) trying to paint it as ISIS (granted ISIS did claim this, with no evidence, but that likewise didn't stop radical right wing people from immediately jumping to this as well) or an extremist liberal, and I'm sure it won't be long before we get claims of false flag (since after all Newtown didn't work, so they had to go to fully auto to prove that they're this deadly so they can take all guns away). Sadly, the people claiming this type of stuff are actually the ones that end up enabling/supporting it (often not directly) as seen with Timothy McVeigh (want to talk about a situation where he almost absolutely had help from an entire network of people who were intent on making such an attack come to fruition).
Not saying its impossible for him to have had help, just saying there isn't necessarily anything precluding him from acting alone. So please don't feed into speculative craze.
That's the issue. Because of how regulations (or lack thereof) currently are, it enables singular people to amass weapons cache with effectively little to no actual official oversight. People buying up a lot of weapons and ammo should be under scrutiny and show what their uses for it are (and should be required to track and show proof of where and how it was utilized). If they are hoarding it for "the zombie apocalypse" or legitimate use (shooting range, personal enjoyment), it shouldn't matter as long as they show they're being responsible about it (with notes made when they're claiming stuff like "defense for when society collapses" or "when they come to take our guns"). This won't stop mass shootings, but extra scrutiny will help. And by stopping open unrestricted sales it will help restrict gun availability. Again, it won't put an end to plenty of these shootings (which often the gun purchases are fully legal, and typically aren't exploiting private sales loopholes), but tightening up general availability and demanding more responsibility will help.
I really think a sound solution would be for police to have ATF qualified person (at every single department/precinct/etc), who basically just keeps tabs on this stuff. Someone known in the community, who will put a human face on it and make it more routine. So its not a group in ATF jackets showing up out of nowhere, which immediately makes situations tense (leading to stuff like Ruby Ridge/Waco). Someone that could visit people with large amounts of weaponry, and just make sure there's nothing iffy going on (say, contact people ahead of time, go unarmed and able to make personal judgement calls so that it isn't just about citing and handing out tickets; but also someone that knows their shit and actually checks things). It also would help community outreach for the police, where it can hopefully make things less fearful for them so that they can approach situations better (for all involved), as they can have frank discussions about carrying and handling guns especially during police stops. Again, no this won't "fix" mass shootings, police shootings, shootings in general, but it will help.