A manhunt is under way for a man described by officials as extremely dangerous after he shot and killed grandparents who took him into their Renton home after his release Friday from prison.

King County Sheriff John Urquhart said Monday that 26-year-old Michael "Chadd" Boysen has threatened members of his family and law enforcement. Investigators say he searched the Internet for gun shows.

Boysen is white, 5-foot-7, about 170 pounds. He may be driving his grandparents' car, a red 2001 Chrysler 300 with Washington license 046XXU.