I want to float a theory and want you guys to tear it apart, poke holes, etc. What if the person responsible was very near to where some of the girls lived? What is that person to have done some pretty rough things in the past and maybe in the present but you overlook it because they are your neighbor and "just don't seem like the kind of person that could do something like that." But they are familiar to the girls and the things going on in the lives of some of the families from proximity and time spent? And then what if that person was involved in some significant way. It would easy to lure girls into a car because this person is know and we've known him all our lives.
After the girls disappear, what if that person who likely had work, hobby and living proximity made it no secret of being involved and in fact threatened surviving family members and people close and told them what the person would do if anyone talked. It wouldn't just be the person that talked that would be dealt with, it would be family members, friends, etc. And if this person was known to have killed and disposed of before, what's to say that people ran and other started acting fishy, messing up timelines and clamming up because they were afraid to talk. They were afraid for their lives and it was real imminent danger both closely and with a network that could make bad things happen?. What if every action was governed by the fear of losing a spouse, another child, a friend, a neighbor, etc.?
Does that explain motivations? Does that capture enough data points? Is it viable?
Again, poke holes and tear it apart.