I don't think that the investigators were thunderstruck and suddenly knew that Jessica was abducted. I think that, given that Lyric and Elizabeth were still missing, an 11 year old had recently been abducted and now there was another young girl missing, all within a few hours drive ... police weren't fooling around.
The disappearance of Jessica was immediately treated as a stranger abduction. No one decided to have a 1000 people ground search, no one drained the lake near the school ... it was understood as a stranger abduction. That should have happened in the abduction of Lyric and Elizabeth as well. A second line of investigation could have been whether they wandered off, went for a swim with the leeches, fell into a lake when they should have been less than a block from home, a third line of investigation could have been whether the parents had a bad association. It was backwards in Evansdale ... perhaps because things like that don't happen in Evansdale.
It was also the perfect staging of the bikes and purse, each on either side of the fence, that perfectly predisposed police to assume that the children were in the area.