You could contact the police agency who investigated the death and see if they are willing to reopen their investigation based on the information that you have showing that the case is a murder and not a suicide. You could also contact the Medical Examiner or Coroner who investigated the death and see if they are willing to reopen their investigation.
A few months back around January 2011, there was a widely reported case of a family who succeeded in asking the court to order the coroner to change the cause of death from suicide to something else.
From what I understood, there is a little known and used statute that allows certain persons to bring a suit in superior court to dispute the findings of the coroner.
You can search the archives of the only major newspaper in Seattle for the articles. Search for the words "coroner suicide court".
So, in WA, it is theoretically possible to ask the court to hear whether the coroner's finding is correct. Whether you would succeed depends on your facts.
You should review your specific facts with your attorney to see what your legal options are.