Sgt. Cindi West says the King County Medical Examiner's Office determined the cause of death for 82-year-old Robert Taylor and 80-year-old Norma Taylor.
Authorities have accused 26-year-old Michael Chadd Boysen of killing the couple after they hosted a party for him Friday to welcome him home from prison after he had served a nine-month sentence for burglary.
Boysen's mother discovered the bodies Saturday evening. She had been called by a family member who became concerned that the couple hadn't answered their door.
In comments made to a King County sheriff's spokeswoman and released Thursday, Boysen's mother, Melanie Taylor, says although her 26-year-old son spent time in prison for burglary and robbery, he never threatened his family and the family never felt threatened by him.
In Taylor's words, "when it came to Chadd, they saw the good in him and were forgiving of his choices."
The motive for the killings remains unknown, West said.
"Between the family and detectives we have no idea," Taylor said earlier. "It's just bizarre. The family loved and supported him the whole time he was in prison."
Boysen was arrested Tuesday after a daylong standoff at a motel in Lincoln City, Ore. Officers who forced their way into his hotel room found him lying on the floor on his back with apparently serious self-inflicted cuts, Lincoln City police Chief Keith Kilian said.
He was flown to Legacy Emanuel Medical Center in Portland, where his condition has been upgraded from critical to serious. West said he is conscious, but it was not clear if he is cooperating with authorities.
It could take a month to extradite him to Washington state, West said.
Boysen was previously in prison in Washington between 2006 and February 2011 for four robbery convictions. Those convictions were related to an addiction to narcotic painkillers, said state Department of Corrections spokesman Chad Lewis.