In my opinion anyone with credible information who knew that it was credible would have come forward by now.
I used to think that, but I can see why some people wouldn't.
Anyone in an "awkward" situation (having an affair, for example), to various degrees of illegality (someone without valid papers); someone who'd commited a crime of some description); someone who'd personally done nothing wrong but just had a suspicion about a friend, family member, employer; someone who was simply terrified of being stitched up by simply reporting a suspicion or evidence, or even didn't want their name leaked for other personal or professional reasons with reporters doorsteppinng them and their names splashed on every tabloid.
Media leaks certainly can't have helped. Nor is the uncertainty of whether one could be prosecuted for an unrelated dismeanor that could have come out in the wash.
What would happen to someone who was, e.g., an unwittinng accomplice? How could they know whether they'd face prosecution or not?
A few years ago, a teenager died during a party at someone's house as a result of drugs. The other kids clammed up. The police publicly assured them that they wouldn't be prosecuted for having also taken illegal drugs, but simply wanted them to come forward with any information. I can't remember much more about the case, but I think that some of the teens did actually come forward in the end.