"Huge cache" from the OP and "Arsenal" from the article...I read those and as I read the article I had to laugh. If the article is accurate it appears that they had at least 4 weapons and most 6 weapons besides the BB gun the article mentioned...another lol. If this constitutes a "huge cache" please don't come check my safes out. My modest collection of weapons might induce a coronary if six guns is a "huge cache."
Also, I still use a pressure cooker like the one shown in a post above to can vegetables out of my garden and fish out of the creek (canned carp is every bit as good as canned salmon) and yes, I bought it at a yard sale. I just thought the comment along with the picture was funny ... and a bit ironic.
As to the guys getting the guns, maybe they passed background checks just fine. I don't know. Maybe they bought them from individuals or had them provided. Maybe the tooth fairy left them for them. The point is, no matter the efforts and good intentions of some, bad people are going to find ways to do bad things. No amount of hand wringing or passing knee-jerk laws is going to stop that.