A bit of (amateur) thinking out loud…
My strongest speculation all along has been that Abby was catfished online, and she intended to meet up with a cute teenage boy in a remote area. Libby, her faithful friend, came along for support and safety. We know the outcome.
I 100% believe this was well planned ahead of time. It appears the Perp came well prepared with his murderous tools under his jacket, stuffed down his pants, and in that fanny-looking pack. Perhaps he had a gun, stun gun, knife, or hatchet. I believe there were other “tools” as well. Seems like he was accustomed to carrying gear, such as a hunter would.
He was confident, knew the area well, and had conducted dry runs. The area he picked must have worked from an acoustic perspective, and he would have checked that out ahead of time as well. No one heard their crys, or at least that hasn’t been reported in the press. The sound did not carry, and he knew it wouldn’t.
So how are they connected? Clearly he knows the area very, very well. How did, and why did he connect with Abby if my theory is correct? i’ve heard some reports that Abby didn’t have a phone or wasn’t active on social media. Baloney. All kids that age figure out a way, and use programs their parents have absolutely no idea about whatsoever. They use friends phones, library computers, whatever. They are sneaky, and tech savvy. They cross their tracks well.
I think this is someone loosely tied to her family that doesn’t live in town, but knows the town well. LE has suggested this as well. I believe they were communicating, and arranged to meet. We know the rest.
I would think about anyone even loosely attached to Abby’s family that always paid special attention to Abby. Gave her little nicknames, brought her gifts/candy, teased her, tickled her, gave her piggy back rides, and so forth. Handsy. That’s where Id look. The one everyone says - "Gee, Sam is so good with kids!" That guy. (No offense to the Sams of the world).
Of course this is all amateur opinion and speculation!