They're resorting to Billboards yet they believe this is an isolated incident. Huh.
They said Leila Fowler wasn't an isolated incident, and still had the community looking for the supposed intruder. They also said the brother was a witness. Yet, he was arrested...and I don't think they ever thought there was really an intruder.
I'm not suggesting that the family is involved, I'm just saying LE might have a good idea who they are looking for. It's not uncommon for LE to squeeze a suspect from all sides.
Also, the could simply mean it's "isolated," in that it's not connected to any other crimes. They could have no idea who did this, and still think their presence is enough in this community, that another child won't get snatched right now.