Aside from investigating what tips they get, I've thought for a long time that LE is mainly playing a waiting game for one of two things. One, someone gets arrested somewhere and the DNA hits, or two, someone with knowledge of the crime decides to talk. They've decided that nothing else they release would help with either of those, so they just stay silent.
That said, I really doubt that there's NOTHING else they could release that would not compromise the investigation. They just don't think it will help it either. I was talking about this case with someone the other day, lamenting that LE wouldn't say anything new, if for no other reason than to drum up major new coverage of the case. They pointed out that this case is now so well known that it's hard to believe there's someone out there with crucial knowledge, who doesn't realize what they know is important. I thought that was a good point, even if it's pretty frustrating.
