My thoughts on this case have always been that J was involved with another man. Perhaps I have looked into this too much, but a lot of the details that came out made me suspicious:
I thought it was strange someone knocked on her door while she talked to her dad on the phone, but said “it wasn’t important” or something along the lines of that. Did she have friends who would come over? The fact that she told her coworker “she doesn’t date coworkers” was odd considering she was in a relationship, she could have said that instead? Her ex being in the same town is strange too. The cops telling her parents “you don’t know her as well as you think.” Just a lot of very odd things.
None of this is said to judge her, I don’t know the particulars of her relationship. But a lot of the info/quotes that I’ve read makes me think she was seeing someone else and maybe they were responsible.