The police have remained impressively firm in their decision to not answer many basic questions and intend to release their evidence all at once in a final report down the line. This may be the proper approach, although it can frustrate the media and certainly opens up speculation by citizens. But they can't say everyone shut up until the report.
So I offer my speculation. I base it on tidbits that media personnel claim to have sniffed out, or by direct witness testimony to the media. None of this makes any single item absolutely true but there is a possibility, and more than a slight one, that the situation we are dealing with is as follows..............
The mother was aware of problems that her son was having. What she was noticing over time grew to the stage that she felt the need to start thinking of having her son committed. And said that to him, angering him. During this period that the mother was observing the problems increase, one or more of the mother-decided bonding outings with her son was trips to the gun range--or maybe, worse still, his individual trips to the gun range condoned by her. From the point of the mother deciding to possibly commit her son and the shooting is a time frame of undetermined length as of now. Whatever it was, she spent the last two days out of town before the shooting.
So we have the mother considering committing her son who has growing to the point of serious problems. She sees his bad reaction to that. Then she spends time out of town with him in his, by this time, regular basement habitat. With the household arsenal kept in a container in that basement. Where the walls are covered by pictures of war, violence, and guns.
The term "negligence" would not seem to be strong enough here, if these are the facts. Whatever are the proper word or words that would adequately describe this situation, if there are any, I submit that a profanity or two added for emphasis, is a must to even get it in the ballpark. And a word or two about stupidity as well, almost rising to the level of being complicit.