You can certainly try to get the police interested in pursuing it but realistically there isn't much that can be done. The courts
have rule that the decision on whether or not to pursue a case resides solely with the police and DAs office and you can't force them to do anything.
A few things you could try:
1) Go through the chain of command at the police department asking for answers. You can do this yourself or, for added pressure, hire an attorney to do it for you.
2) Contact local reporters and see if any of them are interested in pursuing the matter. Reporters are always looking for ideas for stories and this attention sometimes causes the police to suddenly become more interested in pursuing a case.
3) Contact the ACLU and see if they can be of assistance. It would be unusual for them to get involved in this type of case, usually they would only get involved if there was some indication the police were involved in the actual death and not just a messed up investigation
4) Contact your state's Attorney General or the US Department of Justice
and inquire as to whether they would be willing to pursue it. They will occasionally get involved if they have heard of other, similar cases involving that department. It is worth contacting them because you wouldn't know if there are other complains of similar conduct and so the worst that can happen is they tell you they are not interested.
These are the most effective ways of dealing with the issues and are also ways that are either free or relatively inexpensive.