SEATTLE — The grandparents of a man accused of killing them were strangled at their Renton home after they held a party to welcome him home from prison, the King County sheriff's office said.

The King County medical examiner's office determined the cause of death for Robert R. Taylor, 82, and Norma J. Taylor, 80, was strangulation. Their grandson, Michael Chadd Boysen, 26, was arrested Tuesday after a daylong standoff at a motel in Lincoln City, Ore.

He's in serious condition and conscious at Legacy Emanuel Medical Center in Portland, said sheriff's Sgt. Cindi West.

The co-manager of the motel said Boysen seemed normal. He was friendly, chatty, paid with cash and showed his identification as if he had nothing to hide.

Boysen said he was on a road trip when he checked in Monday, said Leah Kallimanis, the co-manager at WestShore OceanFront Suites.

It wasn't until early Tuesday morning, as she and her husband and motel co-manager, Adrian Kallimanis, were watching TV news and going over the previous day's guest log that it hit her.

"All of a sudden my wife said, 'Oh my gosh, I checked that guy in yesterday.' She looked at the registration form and the name and she said, 'This is the guy who is on the news right now,'" Adrian Kallimanis told The Seattle Times.

They called police, which prompted the standoff that ended with officers storming Boysen's room and finding him lying on the floor with self-inflicted cuts.

The King County prosecutor's office is working on charges, but spokesman Dan Donohoe could not say when they'll be filed.

Boysen is held on a no-bail warrant from the Washington Department of Corrections for violating terms of his release.

He could be returned to Washington in a matter of days. It could take weeks if he fights extradition, Donohoe said.

It could not be determined if Boysen has a lawyer.

It's still unknown why he went to Lincoln City or what could have motivated to kill his grandparents, authorities said.

Investigators believe the Taylors were killed Saturday. Their bodies were found by Boysen's mother when she went to check on her parents.

Detectives have determined that Boysen checked in under his own name and spent Saturday night at a motel in the south Seattle suburb of Tukwila, not far from his grandparents' home, West said.

Oregon State Police found the Taylors' missing car Tuesday in a Wal-Mart parking lot in Salem, Ore. West said Boysen paid cash at a nearby used car lot for a used Ford Taurus — the same car Boysen drove to Lincoln City.

King County Sheriff John Urquhart called Boysen extremely dangerous because of threats he had made while in prison against his family and law enforcement. Authorities didn't learn of the threats until after he made the news, suspected of killing in his grandparents.

The Taylors had picked him up from the Washington state prison at Monroe. They drove him to a meeting with a parole officer Friday, helped him get an identity card from the Department of Licensing and held the party for him.

Boysen had just finished serving nine months in prison on a burglary conviction, said Washington state Corrections Department spokesman Chad Lewis. He was previously in prison between 2006 and February 2011 for four robbery convictions. Those convictions were related to an addiction to narcotic painkillers, Lewis said.