I fail to see how LE can determine the perp is likely known to Mollie, via pings, which seems to be the most part of their evidence, if any. If they had isolated a person known to her whose pings lined up with hers during crucial timeframe...he would be “in the docks” so to speak, explaining himself like WC. Especially if he had been interviewed and claimed to be elsewhere...which should cover everyone known to her who has been interviewed, as if they said they WERE with her, well there he is.
Most people are harmed by someone known to them...far more than by serial killers or passing strangers, so it is a logical assumption that she knew him. But they cannot know this until they have their perp. And they don’t or we would be seeing WC-like activities or an APB for his location.
Jmo