Our not knowing anything more about Alec, one of the two main investigators, is typical of the charm and power of this series, which doles out clues in a miserly way that makes the characters all the more provocative.
For example, we see Susan Wright - after displaying a nasty attitude to the police - opening a closet which contains what is apparently Danny's skateboard. Presumably her attitude arose from her not wanting Alec and the police to look around on the inside of the house by the cliff and finding the skateboard. Her allergy to police serves as a good pretext for not inviting them into the house. But does this make her the killer? Possibly, but likely not.
And we learn that Chloe is involved in some kind drug business - having nothing to do with Alec's problem, but likely having some connection to Danny's killing. Or maybe not - maybe this is a red herring. After all, it has less literal connection to Danny than his skateboard.
So we're left with our full slate of suspects, with a bit more suspicion cast on Susan and Chloe (or her friends and associates), but no winnowing out of anyone. About the only people beyond all suspicion are Ellie, Alex, and the Guv.
Hey, in that sense, Broadchurch is a big departure from The Killing after all.
See also Broadchurch: Powerful Viewing