My first official post of a theory. Not of what happened to the girls, but the actual investigation.
1. LE has DNA, but does not divulge, or even unequivocally confirm this. By doing this, they are hoping that the Killer will be sloppy and they will be able to collect. The killer knowing that a great match to DNA is in their investigation, they will be more likely a little bit extra careful.
2. LE has finally received the results from both Parabon, and familial DNA. They have a Parabon sketch, but will not release it. Why? This would also tip their hand on having this info. Once they receive the Parabon details, they realize who it is, and also realize they had a sketch from early on of the person sighted there(which they had initially discounted). They now know which family (maybe even specific to the person), but are requesting further tips to see this case through court. Furthermore, the PC was meant to presumably say to the killer: "We know who you are, you now know we know, just give in before we arrest you". This would completely avoid a trial and risk any chance of case not sticking.
Just MO.