Investigators have identified the last known remains linked to the Green River Killer, one of the most prolific serial killers in American history, concluding a decades-long effort to identify each of the 49 women and girls he was convicted of killing.

The remains of Tammie Liles, who was 16 when she disappeared from Seattle in 1983, were named Bones 20 when they were found in 2003 because investigators were unable to confirm their identity. DNA testing recently helped investigators confirm that they belonged to her, resulting in all 49 victims being found and identified, the King County Sheriff’s Office said on Monday.

Gary Ridgway, who was known as the Green River Killer, terrorized King County through the 1980s and ’90s. Some of the victims’ bodies were dumped in the river south of Seattle that lends the case its name.

In 2003, he was sentenced to life in prison without the chance of parole for killing 48 people. In 2011, he pleaded guilty to his 49th murder. In the end, he confessed to 71 murders, and some investigators believe he killed more people.

Mr. Ridgway, 74, is serving a life sentence at Washington State Penitentiary in Walla Walla.

He led investigators to Ms. Liles’s remains in 2003 near Kent-Des Moines Road, where investigators found several bones and some teeth but no skull or major bones. They took a DNA sample and uploaded it to a national law enforcement database of missing people and unidentified remains, but were unable to find a match, labeling her as Bones 20.

Ms. Liles was originally identified as a victim in 1988 based on a separate, incomplete set of remains found in Oregon in 1985, the Sheriff’s Office said. Two decades later, the Sheriff’s Office contracted Othram, a laboratory in Texas that specializes in DNA forensics, which in 2023 was able to help link Bones 20 to Ms. Liles.

“I was very elated that in this case, which has taken over 40 years, we were able to put a name to these bones,” said Eric White, the Sheriff’s Office spokesman, adding that he was relieved that officials were able to give the family members of the victim “some closure” from their loss.

“It’s not a good thing to lose a child no matter what age,” he said. “I would have to assume that it was a traumatic experience to hear the words of the detectives.”

Ms. Liles’s family could not be immediately reached late Monday.