Occum's Razor (IMO): This was an opportunistic crime by either a local or by someone from a neighboring town. He saw the girls early on, and started following them. He is very familiar with that particular area of the trail and the crime scene. He led them to a place that was enclosed, raped and killed both of them, and dragged them out to the crime scene some time between midnight and the next morning when the search resumed.
This psychopath is hiding in plain sight now, calm and perhaps even involved in the situation following discovery of the bodies. He wants to be "helpful". He's a quiet person...no prior record. FBI suspects him, but is waiting for DNA to confirm.