I think you are correct in saying "not enough proof" and that's why no one has been arrested. There was also the possibility of a preconceived POI that perhaps some things were not investigated completely.

In the decisions made by LE in regards to the case early on I believe one person was questioned and given the polygraph and meanwhile other developments were happening that perhaps would have pointed another way. This is only my opinion...

Maybe they did have an idea of who did it but just not enough evidence to warrant carrying it further. My guess is that their investigation continues but the paperwork might be starting to disintegrate if not properly stored and it would be a shame if that happened. There are archival processes to safeguard against any possibility of that being an issue. I'm hoping MCPD doesn't let that happen..