There was an FBI agent participating in the search while Abby and Libby were still missing.
It is mentioned at this press conference on February 22, 2017 at the 13 minute and 9 second mark.
I would like to call it a stroke of good luck that there happened to be an FBI agent in town that day and that is what brought in the FBI into this investigation so early on.
If there was something the killer did that made it obvious that it was a SK, then possibly whatever it was is linked to Lizzie and Lyric? The similarities to that case are eerie.
It could explain the 6,000 billboards in 46 states and the radio ads that happened so quickly. Perhaps LE did not want to create a panic situation about a SK so they put those up 'quietly'?
Or not.
JMO, speculation, etc.