The more I read about this the harder it is for me to believe that this was anything but a targeted killing that got out of hand. I don't think it has anything to do with a cult, political extremists, drug dealers, or anything like that. All of these kids, the victims, and the surviing room mates seem like nothing but kind, happy, well-adjusted college students. I'm going to stick with this being done by some obsessed kid that knew her although she may not have even known he existed. I'm gonna go with male, 18-30, socially awkward, possibly a U of I dropout. I don't think it was sexual or a thrill kill. I think it was anger that drove him.
But that may make him very hard to find. He likely has no close friends that would know of his obsession or notice any change in behavior. He will probably quietly leave town. His DNA is probably all over the crime scene but he probably does not have a record so nothing to match. They might get lucky with familial DNA matches but that could take years. And there is probably going to be so much foreign DNA on all of their bodies that there will be a lot of false leads. All the the victims were around lots of other people that evening in a crowded bar or at a party. Plus, all the visitors to the house!
Think about it. If you knew someone who was obsessed with one of those girls, now or in the past, wouldn't you have already contacted LE?