Possibilities.
1. Gang Initiation. Nothing is inconsistent with this being a gang initiation. The fact that the suspects are females proves nothing. There are female gang members, and they get initiated. It should be investigated.
2. Hate Crime. Hate is on the rise in the US. I would not be surprised if this turned out to be a hate crime. It should be investigated.
3. Kidnapping for sexual assault and torture. I see nothing inconsistent, with this being a sex crime. Again the fact that the suspects are females, should not eliminate this possibility. There could be suspects involved, other than just the two females. It should be investigated.
4. An acquaintance with a vindictive. They can investigate it, but I doubt they will be able to find any suspect who would have a motive to do this to her. Some person who she might have had issues with in high school, is unlikely to still be holding enough of a grudge, to come back and kidnap her and torture her, 15 years after the fact. That person would have to have some really serious mental issues, to do that. I hope they don't waste too much time investigating the possibility.
5. Human trafficking. I know this is going to PO a lot of people, but human trafficking is a non issue in this country. It makes great movies and books, but in real life it doesn't work. It's too easy for LE to break up any such rings. LE in the US is constantly conducting stings for prostitution.
The examples given in this thread for human trafficking, are mostly teenage runaways who get involved with pimps. Which is totally irrelevant to this case. I hope they don't waste a lot of time investigating the possibility of human trafficking.