PORTLAND, Oregon (Reuters) - Police ended an eight-hour standoff and stormed a motel room in Oregon on Tuesday to arrest an ex-convict accused of killing his grandparents after they had thrown him a party to celebrate his release from prison, authorities said.

The suspect, Michael "Chad" Boysen, 26, was taken into custody at about 7 p.m. local time suffering from self-inflicted knife wounds, Lincoln City Police Chief Keith Kilian told Reuters. Kilian said he did not immediately know Boysen's condition.

Boysen is suspected of slaying his grandparents in Renton, Washington, and stealing their car. Their bodies were found on Saturday, a day after he was released from prison where he had served a sentence for a burglary conviction.

(Reporting by Teresa Carson; Writing by Steve Gorman; Editing by Eric Walsh)