On the face of it sounds like a terrible investigation, but that's really tough to guarantee without access to all the facts.
But, everyone has a boss and you should systematically take it up the chain of command. Contact the state rep would've been my six or seventh stop if it's a issue in a fairly good sized city. If you've gotten bad answers all the way up thru the police chief and/or commissioner then see if the department has an internal affairs division. If that doesn't work go to city council, then the mayor, then the state representatives and senators, and then the governor if you have to.
I'd get the local media involved as soon as I had yo go outside the police department for resolution.
There is another route, but it's expensive... there are private investigators that can do this sort of thing and depending on their findings you could file a civil suit against the faulty driver and possibly even against the "non" investigating police department. Private investigators and lawyers are never cheap though.
I'm very sorry for your loss and I hope and pray the authorities do everything they need to do to hold the right people accountable.