Sorry to hear of your loss. Contact an attorney and find out if an autopsy was conducted. Suing a person for wrongful death is not easy. Need to show they are liable and more importantly that she has insurance or assets. If it was a crime, then insurance company may disclaim responsibility.
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Consult a lawyer. Generally though you can't sue the police for how they conduct an investigation and you can't sue the girlfriend for wrongful death unless she killed him or failed to get help if she had a duty to do so and the help would have prevented the death.
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lawyer might be helpful, but you may want to cotnact the prosecutor's office if you beleive that there was foul play. However, do you think she killed him or this is simply drug abuse where she got rid of the evidence before calling the police.
It does seem clear that it is likely that your son died from some type of drug abuse. What makes you think that she is responsible? Do you really want to add to the tragedy? What would he have wanted?