Since he had no previous criminal record ( at least haven’t heard of one) they could have matched up the DNA via all those DNA heredity programs out there.
RULE 1 - If you plan to be a serial killer do NOT submit your DNA to ancestry.com
What you speak of is an ancestral group match.A DNA match proof at the crime seen they need a sample of his own DNA to match. For then to get that sample as admissable evidence it has to be by the accused consent or by a court order.The accused is not required to give evidence against himself.
That’s why I am saying the evidence against the accused is circumstantial at this point. That may change after his extraditio to Idaho, we will see.
BUt so far there is no evidence connecting him directly to the crime scene that can be adduced as admissible evidence on court.