The Sheriff told that story about the black bag, on the basis of an interview with the father. Unfortunately the Sheriff is not reliable, but it seems likely that that’s what the father told him.
Did the father know what was in the black bag? Who knows? But why would he chase after the kid into the desert, unless he suspected something pretty bad?
I agree, the father must have known something. The kid was leaving notes in the safe and who knows where. That it was also reported that the parents cried in the street for and hour after hearing the news kind of fits with having prior knowledge of his intent. Unbelievable that they wouldn't have reported him and just remained in denial hoping he wouldn't do anything. I grieve for them having a son that age myself but you gota do what you gota do. Damn!