King County prosecutors have charged a man accused of killing his grandparents at their Renton home with first-degree aggravated murder, which carries the state's ultimate penalty.

Michael Chadd Boysen was arrested March 12 after a standoff at a Lincoln City, Ore., motel and had been held at the Multnomah County Jail in Portland.

The Department of Corrections says Boysen arrived in King County on Thursday.

King County prosecutor's office spokesman Dan Donohoe says prosecutors now have 30 days to decide if they want to pursue the death penalty or a life sentence.

Boysen is accused of strangling his grandparents after picking him up from prison on March 8 and welcoming him home. Prosecutors filed two charges of first-degree murder.