Quintuple Mass-murder in Albquerque NM

It is being reported that 15-year-old Nehemiah Griego, nephew of State Senator Eric Griego, murdered his father, prison chaplain Greg Griego, and four other family members, three of who were children.  An assault-weapon style rifle appears to be the primary weapon that was used.  Reporting of this last fact immediate drew the ire of lunatic gun nuts everywhere, who predictably trotted out every straw man fallacy in the book such as, "guns don't kill people..." and "you could kill someone with a tennis racket, so let's ban that and not the device that was actually used".
At least it will be difficult to argue that this kid killed because of lack of the Ten Commandments being taught, given that his father was a preacher.  Naturally, some other bogeyman, like violent movies will have to be blamed, even though they have the same violent movies in Canada and Australia, but don't have as many gun massacres, due to stricter GUN REGULATIONS.  How was this 15-year-old able to obtain ready access to a rapid-firing, high power assault style weapon?