Did the dad ever say what the officer and Brian talked about? Over an hour of chit-chat with a guy you found naked in a field? I feel like there would be something substantial in that conversation that would offer a tip on Brian's thought process at the time. Did Brian mention religion to the cop?
He'd been on a religious awakening for awhile, but how long did he attend the Baptist church someone mentioned? Two visits, two months of visits, etc.? Had he made friends at this church?
Where was Brian from and had he always lived in AZ? The dad lived elsewhere (though Brian was in his mid-20s so not that you'd stay near your family), so what were his ties to AZ? What had prompted him to move there or did the dad move away at some point?