C'mon, they arrested a guy because they think he did something to her. I don't think they arrested a guy because he helped cover up a crime and still don't know who the real perp is.
So this guy harmed her and a woman (maybe a girlfriend, wife, mother?) helped him hide it.
Random speculative theories:
1. She was having an affair with a police officer and he was in another relationship, maybe married? Wife finds out, they conspire to hurt Sarah? Or he hurts her and the wife/SO helps him cover it up?
2. The police officer preyed on Sarah. He used his position of power to get her into his vehicle and then harmed her. The woman (a wife or girlfriend or maybe a mother?) helped him cover up the crime in some capacity.
3. The police officer is in fact somehow related to or an acquaintance of someone known to Sarah. Not sure what the motive would be here but maybe this is the connection, who knows.
Edited to remove some stuff so we don't get the thread shut down