Ridgway, who has been dubbed the Green River Killer, already confessed to killing Rebecca "Becky" Marrero in 1982 as part of a 2003 plea deal that spared him the death penalty. Marrero, a 20-year-old mother, was last seen when she left a motel in 1982.
Prosecutors declined to charge Ridgway at that time because he was not able to provide conclusive evidence that he killed her, but the plea deal required him to plead guilty to future King County charges based on new evidence.
Marrero's remains weren't discovered until Dec. 21 when teenagers found a skull in a ravine at Auburn, south of Seattle. Ridgway was charged Feb. 7. He was brought from the Washington State Penitentiary at Walla Walla for the arraignment.
Ridgway, who was a commercial truck painter, has been convicted of 48 murders and confessed to or been suspected of dozens more. He preyed upon women and girls at the margins of society -- runaways, prostitutes and drug addicts strangled in a spree that terrorized Seattle and its south suburbs in the 1980s. Several victims were dumped in or posed along the Green River.
He was arrested in 2001 after advances in DNA technology enabled authorities to link a saliva sample he gave authorities in 1987 to some of the bodies. He pleaded guilty two years later, agreeing to help authorities locate as many remains as possible.
He is serving life without release in solitary confinement at the state prison, where he's allowed out of his cell one hour a day four times a week.
Marrero's remains were found 100 feet from where investigators found another of Ridgway's victims, Marie Malvar, in 2003.