I feel like they were killed where they were found. I base this on simply the girl's weight and size. I know Lizzy was small, but didn't Lyric weigh close to 140 lbs? If so, it would have been nearly impossible for one person to carry her very far. That is an extreme amount of weight for anyone or even any two to carry, especially in rough terrain. Now, having said that, they may have been taken somewhere else first and things done to them, but I can't help but believe they were killed where they were found.
I don't know if this area is assessible by 4-wheel drive or not. I believe I saw a video where a vehicle was able to get somewhat close to where they were found, but noy all the way there.