I guess that leaves with with 2 options.
1 - hire a lawyer to get more answers from the police
2 - hire an PI to dig even deeper.
It is very sad and probably expensive, but it appears no one is taking this seriously right now.
I think the PI might be best route. at least they should be able to gain access to official documents or get an official explaination.
once they know you are serious, it does change things.