May I respectfully suggest that in some murder cases, there may be reasons why those close to the case are left devastated, but would perhaps have reason to not want the murderer known publicly and/or not want him punished? There could any number of reasons, such as fear of retribution, fear of someone who links back to you or your family, etc.
It's very hard to imagine, but if I stretch my mind a bit, I can imagine situations where I'd perhaps be gutted, utterly devastated at the murders, but have reason to not want the murderer caught, especially if I thought it wouldn't happen again. There's earthly justice, as in the prison/hanging kind, but many of us believe in an ultimate justice. Perhaps, in some situations, that is enough sometimes. I don't mean enough absolutely, obviously, but enough in that person's mind. I wonder.
That person might do much in wanting to keep the memory of his/her family alive, might give interviews, etc., but balk at helping find the killer...