The bodies were discovered Saturday evening by the couple's daughter, who came to the home in the 16200 block of 145th Avenue SE when she couldn't reach anyone there. Deputies and medics arrived a short time later and pronounced the couple dead.
Investigators would only say they don't believe it was a murder-suicide and there are no suspects in custody.
Later Saturday, police said they were searching for 26-year-old Michael Boysen, the couple's grandson, who is unaccounted for.
Sgt. Cindi West initially said authorities were not searching for him as a suspect of the crime, and said his relationship to the incident is unknown - but they know he was at the home Friday night.
West later said a warrant has been issued for Boysen and that he is wanted for questioning in connection with the killings.
Boysen is 5 feet, 10 inches tall and weighs about 170 pounds.
Investigators are also searching for the slain couple's car, which is missing from the scene. It is described as a red 2001 Chrysler 300 with Washington license plate 046-XXU. If you know where the car or Boysen is, you're asked to call police
Investigators have not given any further details about the murders and only identified the victims as 82- and 80-years old.
Updates to follow as more information becomes available