That would be up to the local or state police/law enforcement. It would not likely involve a wrongful death civil suit in any way.
Many state bureaus of criminal apprehension have cold case units that look into unsolved or questionable deaths. I would advise that you need more than just suspicion to turn this agency into this case. You need some new evidence that places into question the cause of death.
A civil case for money damages would be barred by the statute of limitations. A criminal case for murder has no such limitation, but it would be up to the DA and police to arrest and prosecute.
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