I think since the police have zero motive, and his brother stated he never even knew he owned firearms or knew how to use them.
I've been thinking about both of those things too. The brother part makes sense to me, but you have to know more about my family for that to make sense. I have a biological brother. We haven't spoken in several years. If, for some reason, the police asked him about me, they'd get a story that I, and most people who actually know me, wouldn't recognize.... But it would be believable, if you didn't have any other information. One of the things I've read is that their father had a rather interesting criminal history. Apparently he made the FBI 10 most wanted list and was, at one point, referred to as a "psychopath". Now, I don't know who decided that or how, so I take it with a grain of salt, but I find it interesting. I've had more personal experience with that type of person than I like to think about. Growing up in a family with a dose of that? It will have an effect, I promise. Hard to say what and "what" will probably vary from person to person, but no one escapes unscathed.
I don't get the "no manifesto" part. Most of the time, it seems like the people who feel they have the right to commit this kind of atrocity want to make darn sure at least someone gets their point. Honestly, this is one of the things that makes me suspicious of the girl friend. She says she "had no idea"? I'm trying to understand the scenario where your SO is collecting THAT many guns, apparently over time, and you "have no idea". No saying it's not possible, just that it's suspiciously weird. If he actually had an axe to grind against ANYONE, it baffles me that he never vented about it to anyone. Seems unlikely. So, maybe there's more they need to find out or maybe something else.
@Dr.Knowbuddy , at this point, I'd certainly leave your scenarios on the table as possibilities. But, to what end? Who'd actually want this to happen and what are they after? No matter which side of the political spectrum they claim allegiance to, they'd be "The Bad Guys", as far as I'm concerned.