To me, jenandemmie's theory (post #3) is reasonable.
The perps (plural) very well could be a couple of young, video-game playing punks with lots of time on their hands in the summer. They even may have been grooming one or both of the girls with this very plan in mind.
They lured the girls to the lake, grabbed them, and took them to the 7 Bridges place, which they knew from partying there - and which they probably knew would be isolated.
They walked the girls in, sexually assaulted them, and murdered them, right there.
Horrid to contemplate, but highly possible.