This case is getting a lot of support from the FBI and LE... that is good to see. As WS'rs know, there are far too many of these cases and we aren't doing enough to solve them.
In this case my insights lead me to favor the following theories:
1) Perp is someone close to her(personally or proximal geographically) or connected to someone close to her. It is somewhat of a movie type fantasy to think that a serial type killer just suddenly appears out of nowhere, with no one seeing anything to abduct a random girl in a very small town in a rural area. Odds are highly against it. It is most likely that this is someone she knew directly or peripherally, or at least they knew who she was. I think the timeline has some value to possibly rule out some people if their alibis are iron-clad. So far I haven't seen anything released that actually rules anyone out. The window of her disappearance seems fairly wide, by at least 9 hrs. Because of her relationship, where she lived, shared vehicle arrangement, etc... she would have been in frequent contact with multiple people. LE should reasonably be able to establish a much narrower time of her actual disappearance. In this day, even in such a town, someone of her age is going to be using phone, social media, etc... a lot and so if there were some other guy involved there would be traces of that there. <modsnipped> It's not unusual for such a thing to occur. It's in fact, quite common. Rapes occur far more in this type of scenario than total strangers, and that is one reason they are far less reported. IF I were LE, I would really be looking at this closely. What would the motive be to kill her? <modsnipped random accusation>
2) If it's not someone directly close to her or someone she was communicating with, then the theories that it's someone connected to someone close to her make sense. That should be able to be deduced through a lot of legwork. Other possibilities exist for transient contact. For example, was there a Fair or Carnival nearby around that time? Summer is a time for Fairs and Carnivals and it's no secret that people that do that kind of work often come with criminal histories. If she caught someone's eye, they might have pursued that... however we have a similar problem here as with a random act or a serial professional(which are extremely unlikely) - someone should have saw something. To get to her, where she was living, to know where she was and be watching... that would be pretty obvious to her neighbors and locals. Out of town door to door sales people, delivery people, etc... people that would know she was there and could get close enough without raising suspicion.