Had Denise Louise Plager met Gary Ridgway much before October 1983, her son might never have existed. Had she never met Ridgway at all, that son — South Whidbey resident Daryl Imburgia — might have lived a completely different life.

On Wednesday, as Ridgway signed documents admitting his guilt in the slayings of 48 young women, Imburgia was still getting to know more about his mother and how she died. Along with dozens of other family members of Ridgway’s victims, Imburgia was at Sea-Tac City Hall to get briefed on what would happen to the killer and on the future of the investigation of the crimes of the man known for 21 years as the Green River Killer.

For Imburgia, a 2000 graduate of South Whidbey High School and now the 21-year-old father of one, the day put another piece into a lifelong puzzle that had no pieces until he was 14. That was the year that he learned his mother was a Green River Killer victim.

“I had only one newspaper article at the start,” said Imburgia Thursday as he leafed through a thin, three-ring binder that now contains several articles about his mother’s death and the Green River case, as well as photos of Delise Plager’s family members taken during a recent visit with Imburgia.

Adopted at age 7 by a Whidbey Island couple, Imburgia lived for years knowing almost nothing about his past. And even during the years since learning of his mother’s fate, the information has come only in a trickle. Over time and through repeated contacts with detectives working on the case, as well as with Plager’s friends, he’s learned at least a few things about the woman who gave birth to him.

In 1983, the year that she died, Plager, then 22, had intermittent custody of 9-month-old Imburgia, her second child. A known drug user who worked in prostitution on and off, Plager was largely alone while living in Washington state. She’d left her adoptive family in Florida and moved to Washington sometime in the early 1980s. Her son was born after a one-night stand with a 14-year-old boy.

According to Imburgia and his girlfriend, Jaqueline Edwards, Plager met Ridgway in 1983. Recently, Plager’s friends told Imburgia and police detectives that Plager had stayed with Ridgway for an unknown length of time after the now-54-year-old Auburn truck painter invited her to do so. She did not stay long, moving in with friends after meeting Ridgway. But in October of that year, she disappeared.

Plager’s skeletal remains were found in February 1984 near North Bend. She was later listed as the Green River Killer’s 36th victim. She had been strangled to death and likely raped after she died, like many of Ridgway’s victims.

It would be another 13 years before her son would learn of any of this.

When he did, Imburgia said, the information was not an easy burden to carry. He said schoolmates teased him about his mother’s past and how she died.

“It’s a setback,” he said of the effect of his mother’s murder on his life. “It’s so weird being a victim of the biggest serial killer the world has known,” he said.

At the same time, the investigation into his mother’s past has opened some doors for Imburgia. Detectives working on the case assisted him in finding his father, as well as his mother’s family in Florida. A year ago, Imburgia met his biological father, a 34-year-old father of six other children living in Tacoma. Imburgia’s girlfriend, Edwards, said the meeting between the two men went well.

“He said ‘I love you’ and hugged him,” she said.

Speaking about the prosecution of Gary Ridgway, Imburgia said he believes the King County Prosecutor’s Office has done the right thing in not pursuing the death penalty. By keeping him alive and cutting a deal under which Ridgway continues to cooperate with police in finding the burial sites of the killer’s victims, Imburgia said more families will find out at long last what has happened to their daughters.

However, Imburgia said that once all those victims are found, he hopes another Washington county will indict Ridgway for murder, convict him and sentence him to death.

In time, Imburgia said he hopes to learn more about his mother and to tell the people he knows about a young woman who was trying to find her way through a difficult life. He’s started by carrying a photograph of Delise Plager with him almost everywhere he goes. Unlike the blurry police mug shot, which newspapers have used for years to identify Plager, his photo shows Plager — a light-haired woman with a smile much like that of actress Lauren Hutton — in happier times.

In the aftermath of admitting his guilt, Ridgway may be required to apologize face-to-face with the families of his victims. During his killing spree, which lasted from July 1982 to 1998, Ridgway targeted young women between the ages of 16 and 38, some of whom were involved in prostitution.