you need to petition the court in the county where the person last resided.
you will need to show three things 1) they have disappeared or have been continually absent from home for at least seven years without explanation; 2) those persons with whom they would likely communicate have not heard anything from or about them; and 3) a diligent search has been made at their last known place of residence without obtaining information that they are alive.
these particulars need not be proved beyond a reasonable doubt, as would be required in a criminal case - rather, an individual's presumed death need only be established by a "preponderance of the evidence" - that it's more likely than not that the person is dead.
once you have filed the Petition and had a hearing and it is ordered the person is deceased then you can get the death certificate
sorry for you loss - good luck
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