Prosecutors now have 30 days to decide if they want to pursue the death penalty or life in prison for 26-year-old Michael Chadd Boysen, said King County prosecutor's office spokesman Dan Donohoe.
Boysen was arrested March 12 after a 10-hour standoff at Lincoln City, Ore., motel where the manager recognized him from news reports.
He arrived in King County on Thursday after being extradited from the Multnomah County Jail in Portland, state Department of Corrections spokesman Chad Lewis said.
Robert R. Taylor, 82, and Norma J. Taylor, 80, were strangled in their Renton home. They had hosted a family welcome home party for Boysen the night before, after his release from prison in Monroe, charging documents said.
The dead couple were found by their daughter and Boysen's mother. Boysen had agreed to be picked up by an aunt and taken to a drug rehabilitation center.
Detectives said the elderly couple had been found in the closet of their guest bedroom partially dressed in their pajamas. Both had "severely dark/bruised ligature marks around their necks," charging papers said.
A bloody shoe lace remained partially wrapped around Norma Taylor's neck.
By in large, the house remained untouched and there were no signs of struggle, but detectives did find emptied out envelopes that held $5,200. Norma Taylor's family later confirmed that she liked to save and hide cash in her home. Credit cards belonging to the couple, her cellphone and their car were also gone, charging papers said.
Detectives say that Boysen went on a shopping spree, spending around $1,400 at Fred Meyer and Walmart to buy an iPad, a laptop, music CDs, and a pre-paid cellphone.
Investigators also went back to the Monroe prison to interview inmates who knew Boysen.
"During these interviews, I was told that during his incarceration at Monroe, Boysen spoke about being angry at and wanting to kill his grandparents. Boy also shared that he planned to take cash from his grandparents and their Chrysler 300. The theft of the cash from the Taylors had been withheld from the media at the time," detective Christina Bartlett wrote.
Boysen had just finished serving nine months in prison on a burglary conviction, Lewis said. 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.
Last week, Boysen's mother, Melanie Taylor, said her parents always "saw the good" in Boysen and were like "second parents" to him. She said that despite his trouble with the law, he never threatened the family and the family never felt threatened.