By Maria Aguilar Prieto, CNN

Tracy Whitney was last seen leaving a Burger King after an argument with her ex-boyfriend in August 1988. Less than 24 hours later, fishermen found her body in a river near Sumner, Washington, about 12 miles east of Tacoma. She was 18 years old.

An autopsy revealed Tracy had been sexually assaulted and died of strangulation, according to a statement from the Pierce County Sheriff?s Office. Her death was ruled a homicide.

Detectives collected DNA samples from her body that presumably belonged to the killer, interviewed people of interest and administered polygraph tests. No suspect was identified, and the case grew cold.

Tracy?s sister, Robin Whitney, was 11 years old when the murder took place.

Following the loss of her ?protector,? Robin vowed to avoid talking about Tracy ? not to her friends, her father or eventually to her daughters.

?I didn?t want to think about what had happened to her, and what her last moments were like and how scared she must?ve been,? Robin told CNN. ?I?d also have to think about everything we missed out on as sisters, how unfair that was.?

She believed the case would likely never be solved, and thinking of all the ?what ifs? only brought fresh waves of grief and frustration.

But roughly 10 years ago, Robin?s father, who had avoided speaking of Tracy for decades, told her the police had recovered a DNA sample from the crime scene in 1988. The news gave Robin hope. And she slowly began sharing stories of her sister with her daughters

Robin also learned her father had visited the sheriff?s office several times since the murder, hoping to revive the investigation.

In 2005, the sheriff?s office renewed efforts to solve the crime by uploading the DNA sample to the FBI?s national genetic database, CODIS. They decided to resubmit Tracy?s DNA swabs after new technology became available, the sheriff?s office said, but the effort yielded no results.

The first breakthrough came in 2022, after the sheriff?s office received a grant from the state attorney general?s office. With this funding, they sent the DNA sample to Parabons NanoLab in Virginia for genetic genealogy testing.

Thirty-six years after Tracy?s death, the sheriff?s office used lab data to identify John Guillot Jr. as her killer.

Sheriffs, however, could not make an arrest. Guillot Jr. died eight months before authorities could connect him to the killing, Pierce County Det. Sgt. Lindsay Kirkegaard said in a video released Sunday.

How genetic genealogy cracked the case

Genetic genealogy, a combination of DNA analysis and family tree research, finally led detectives to Guillot Jr.

A relatively new field of study, genetic genealogy gained prominence as hobbyists began looking into their family history. It has since expanded into a forensic method used by law enforcement to crack cold cases.

In 2018, it led to the arrest of the Golden State Killer, and it has recently helped solve several other cold cases, including a 1974 murder in Wisconsin and a 1988 murder in Pennsylvania.

Parabon NanoLabs tested the DNA sample and traced the suspect?s family lines to the 1700s. The results pointed to Guillot Jr. as the main suspect, according to the sheriff?s office.

To confirm Guillot Jr. was the killer, detectives had to compare his DNA to the sample they had collected from the crime scene in 1988. That wasn?t possible because Guillot Jr.?s remains had been cremated.

However, the genealogy lab findings led investigators to Guillot Jr.?s biological son, John Guillot III.

The son had also recently died, but the medical examiner?s office had his DNA on file. After further genetic testing, detectives confirmed his father was a match for the suspect?s genetic profile, according to the sheriff?s office.

Detectives believe Guillot Jr. and Tracy did not know each other prior to the killing.

Since Guillot died before he was identified as the perpetrator, his profile could not be uploaded to CODIS to see if he was responsible for any other crimes.

?36 years of locked up emotions?

Robin had been waiting ?almost (her) entire life? to learn the killer?s identity.

But the discovery surprised Robin in other ways.

?The overwhelming support that we?ve gotten from the community that we grew up in, and the general public interest and attention that she?s getting feels like justice in a way,? she said. ?People are talking about her, and they remember her, and they miss her.?

?On the other hand,? Robin added, ?he didn?t face the criminal process, so he got away with it.?

More than three decades later, Robin has the closure she has been longing for.

?Since getting the news that Tracy?s case has been solved, 36 years of locked-up emotions came out all at once. I grieved her as a child, and now I?m grieving her as an adult,? Robin said. ?I?m allowing myself to feel all of it, though. It?s uncomfortable and I?m sad, but it?s an emotional relief that I needed.? (CNN: Text, Excluding Headline)

Update news