Ridgway was arrested in 1982 and 2001 for charges related to prostitution. He became a suspect in 1983 for the Green River killings, and on April 7, 1987 police took hair and saliva samples that were later subjected to a DNA analysis, which provided the evidence for his arrest warrant.
Early in August 2003, Seattle television news reported that Ridgway had been moved from a maximum security cell at King County Jail to an undisclosed location. Other news reports stated that his lawyers were closing a plea bargain that would spare him the death penalty in return for his confession to a number of the Green River murders.
On November 5, 2003, Ridgway entered a guilty plea to 48 charges of first-degree aggravated murder as part of a plea bargain, agreed to in June, that would spare him execution in exchange for his cooperation in locating the remains of his victims and providing other details. In his statement accompanying his guilty plea, Ridgway explained all of his victims had been killed inside of King County, Washington, and that he had transported and dumped the remains of the two women near Portland, Oregon to confuse the police.
Public opinion remains divided as to whether a confessed murderer of 48 people should be spared execution in a state that has the death penalty and imposes it on people who have killed far fewer victims. Deputy prosecutor Jeffrey Baird noted in court that the deal contained "the name of 41 victims who would not be the subject of State v. Ridgway if it were not for the plea agreement," and King County Prosecuting Attorney Norm Maleng explained his decision to make the deal thus:
"We could have gone forward with seven counts, but that is all we could have ever hoped to solve. At the end of that trial, whatever the outcome, there would have been lingering doubts about the rest of these crimes. This agreement was the avenue to the truth. And in the end, the search for the truth is still why we have a criminal justice system.... Gary Ridgway does not deserve our mercy. He does not deserve to live. The mercy provided by today's resolution is directed not at Ridgway, but toward the families who have suffered so much...."
On December 18, 2003, Ridgway was sentenced to 47 life sentences with no possibility of parole and one life sentence, to be served consecutively.
During a two and a half year period in the early 1980s, the Green River Killer is believed to have murdered as many as fifty people near the cities of Seattle and Tacoma, Washington. The remains of two of his victims were later found near Portland. The victims were largely female prostitutes and teenage runaways, picked up along Pacific Highway South (Washington State Route 99), and strangled. Most of their bodies were dumped in and around the Green River, except for two victims in the Portland area.
In November, 2001, nearly twenty years after he had first been suspected in the killings, Ridgway was arrested on suspicion of murder for a number of the deaths, after DNA evidence linked him to three of the victims.
It is known that Gary Ridgway led prosecutors to three bodies in 2003. On August 18, 2003, remains of a 16-year old female found near Enumclaw, Washington, 40 feet from State Route 410, were pronounced as belonging to Pammy Annette Avent, who had been believed to be a victim of the Green River Killer. The remains of Marie Malvar and April Buttram were found in September.
Ridgway confessed to more murders than any other American serial killer. He confessed to 48 murders: 42 of the 49 murders on the police's list of probable Green River Killer victims and 6 more murders, including one as recently as 1998.