My theory (currently): whoever killed the Sandbrook girls also killed Danny Latimer. And may well be a character we've never been introduced to before.
Yes, it's a bit of a crackpot theory, but I don't think that makes it particularly unlikely in the current series of Broadchurch...
Also, I've decided I prefer James D'Arcy in Agent Carter to James D'Arcy in Broadchurch. Although I could repeat that sentence without the phrase "James D'Arcy in" and it'd be just as true...