Here's my theory. Just going to throw it out there and hopefully I'm wrong. It's just a theory. All of this is my own opinion. Young white male between the ages of 25 and 30 who looked to be about 18 or 20 at the time of the crime. Became obsessed with one of the girls in some type of peripheral setting (observed her somewhere), although she was not aware of him. I don't mean romantically obsessed. I think he was obsessed with something he didn't like about her, through not fault of hers.
He found out where she was going to be that day, he had been planning something like this for a while, he hightailed it to the CPS parking lot, made his way to the trail and waited. I think initially someone saw his vehicle but it was common enough that it wasn't paid much attention to.
While I think it was planned, I think he also got very lucky. And while I think he is slightly above average in intelligence, I think he thinks he is a lot smarter than he is. I think, as LE said, "you made mistakes."
I don't think SA was involved or the motive. I don't think he anticipated a second girl being there but she was. I think he has done things in the past that have led him to believe he is already evil in his own mind and since he already believes himself to be evil, he may as well indulge/revel in it. Or he may see himself as some type of a religious avenger, going around all hellfire and brimstone.
He hightails it back to his vehicle and drives as fast as he possibly can to a place where he is seen and known by many people (had gotten rid of his jacket and hoodie and extra pants along the way) parks the car, gets out and mingles. LE obviously knows the time of death and they ultimately determine their POI can't be the guy because he (and/or his vehicle) was seen at XYZ place at 3:45 and well, there's just no way he could have gotten there that quickly. They look into him a little bit more, kick the tires, and someone close to him says, "There's no way that was his Common Vehicle XYZ parked at the CPS building. I actually own it, he drives it sometimes and the vehicle was here all day. It must have been a different Common Vehicle XYZ. They may even 100% believe it to be true, if they were busy doing or slightly distracted something else. I think LE wants the actual vehicle or a more detailed description of the vehicle so they can tie BG to it.
Feel free to poke away at this. Again, MOO, IMO, AF, ETC.