My brother (who used to work under AG Nettles) finally replied to my message asking him what he thought the AG's presence might mean. This is what he said:
Your guess is probably as good as mine. We can probably rule out some things, though. I doubt they have made an arrest in the case. I think we would know about that right away. I also doubt that they have identified a subject who remains at large. No need to wait until next Wed to say that. They would either issue a public advisory to help them find the person, or they would keep it quiet until they located him.
My guess is that they found remains, and they are officially opening a joint state/federal homicide investigation.
It wouldn't be unusual for family of the victim to have their own representation.
ETA:
A more remote possibility would be that the girl has been found alive, and the police wanted things to be kept on the down low until everything was sorted out between her and her family. Again, it would probably not be a criminal situation where anyone has been arrested or identified yet. Maybe a voluntary runaway scenario. The law enforcement agencies involved would come together to pat everyone's back on finding her.
If that was the case, however, I would expect word to have leaked out that she has been found alive.
No reason for the family to keep that quiet until a news conference. If your kid had been found okay, you would let people know.
Odds are, they found her remains.