(CNN) -- A victim of Washington state's notorious "Green River Killer" has been identified 29 years after her disappearance, when a relative saw a TV movie about the serial murderer in April and family members gave DNA samples to investigators, authorities said Tuesday.
The victim was Sandra Denise Major of Rochester, New York, who was a 20-year-old prostitute when she got into a truck on December 22, 1982, and was never seen again, said Sgt. Cindi West of the King County, Washington, sheriff's office.
Gary Leon Ridgway, dubbed the Green River serial killer, pleaded guilty in 2003 to killing 48 women, including Major, though her remains hadn't been identified at the time, West said. The 48 guilty pleas gave Ridgway more murder convictions -- though not necessarily more slayings -- than any other serial killer in U.S. history.
In 2011, he pleaded guilty to a 49th murder, in a plea deal to avoid the death penalty. Ridgway, 63, is serving life sentences without parole in Washington State Penitentiary in Walla Walla, Washington, corrections spokeswoman Judi Feliciano said Tuesday.
"The family said that one of the cousins was watching a movie that re-enacted the catching of the Green River Killer," West said, referring to the two-part Lifetime network movie "The Capture of the Green River Killer."
Ridgway killed mainly runaways and prostitutes in the Seattle-Tacoma area between 1982 and 1998. The first victims turned up near the banks of the Green River south of Seattle, giving the killer his criminal nickname.
"Since (Major) was last known to be in Seattle and ... she was working as a prostitute, (her family) contacted detectives," West said.
Major's family released a public statement thanking investigators "for working together to bring closure to our family."
"We are grateful to finally know what happened to Sandra after all these years," the family statement said. "We were aware of the lifestyle Sandra lived but she was still a part of our family. We last saw Sandra here in New York in 1982. We received a letter from her in 1982 with a Seattle postmark but no return address. We never heard from her again and did not know what happened to her.
"Recently one of Sandra's cousins saw a television show about the Green River Killer. We learned that the remains of some of the victims had not been identified. Since the last contact with Sandra had been the letter from Seattle, we decided to make contact with investigators to see if Sandra was one of the unidentified victims," the family statement said.
Two days after Major got into the truck, her disappearance was reported by a friend, on December 24, 1982, West said
On December 30, 1985, at the Mountain View Cemetery in Auburn, Washington, authorities found remains that were determined this week as belonging to Major, West said.
Also found in the area of the cemetery site were the remains of two other women, though not at the same time that Major's remains were discovered, West said. One of those sets of remains belonged to Kimi Pitsor, and the other remains are still unidentified, West said.
A total of three sets of remains in the Ridgway case have yet to be unidentified, West said.
"If we can get someone who knew the last three to come forward with some missing person DNA, that would be awesome," West said.
CNN's Karan Olson contributed to this report.