I'm not entirely convinced that this was well-planned and executed. A part of me thinks that this was a crime of opportunity and someone just got really lucky. I can see how the two girls might have been exploring Maiden Ln on their own. I can see how it could have happened quickly enough that no one happened to witness it. Maybe someone who knew them saw them venture back there and followed. Maybe that someone was able to convince them that there was an emergency and that they needed to get in a car and go somewhere quickly. They could have said, "Grandma's in the hospital. Just leave your bikes here. We can come back and get them later. We have to go now!" If Elizabeth had already dropped or tossed her purse, she probably wouldn't have bothered to get it. The killer(s) could have snatched them very quickly and without a trace.
That said, people in all walks of life have had connections to the meth world. A few years ago, there was a doctor from Cedar Falls that was making meth or paying someone to make meth for him. I can't remember the specifics of it, but when it all came down this doctor was very connected to the meth world. I've always heard that the best drug dealers don't use their product. I do think that this murder could have drug connections and still have been well-planned and executed.