RSBM
It's been suggested that Rachel was pregnant when she disappeared. While I do not believe she was wilfully cheating on her husband, I do believe she might've been in a situation that got her pregnant involuntarily (possibly with someone connected to LE), and either ran into the man or planned to confront him that day.
I'm not sure where the meeting took place, but it did not go well, and three girls vanished. The car was planted at SS, because that's where the perpetrator wanted people to think the girls had been, and at some point Rachel's family (either willingly or under duress) invented the sightings and started the narrative "last seen shopping at Seminary South"...
If they were responsible for her being used by Mr. X to start with, I can see where they wouldn't want that to get out.
Also, someone higher up in LE or heavily connected to LE would have the connections and ability to get away with murder and intimidate a bunch of kids.
This is just a theory and jmo.