On Feb. 16, 1973, detectives walked through wooded hills at Stanford University, trying to find Leslie Perlov, a 21-year-old law librarian who had been missing for three days.

Even decades later, the officers in Santa Clara County, Calif., could recall to colleagues, through tears, the moment that day when they finally found her body lying on the dirt, a floral scarf tightly knotted around her neck. Ms. Perlov, a woman with a passion for art and literature who had aspirations of attending law school, had been strangled and sexually assaulted, the authorities found that day.

The killing sent a shock wave through the campus and surrounding community, which contended at the time with a series of other high-profile killings that stirred fear among California’s youth. And for nearly half a century, Ms. Perlov’s murder remained unsolved.

But in 2018, shortly after officials re-examined the case, the authorities announced that advanced technology for examining DNA had pointed them to a suspect, John Getreu, who used to live near Stanford and once worked there.

On Tuesday, Mr. Getreu, now 78, pleaded guilty to first-degree murder in the Santa Clara County Superior Court and also admitted that he had sexually assaulted Ms. Perlov.

Mr. Getreu had recently faced a similar accusation in September 2021 when he was convicted in connection with the 1974 murder of Janet Ann Taylor, 21, the daughter of Stanford University’s football coach and athletic director. She, too, had been sexually assaulted and strangled, the authorities said. Mr. Getreu was sentenced to life in prison in connection with Ms. Taylor’s case. He faces another possible maximum sentence of life in prison for killing Ms. Perlov.

The killings, on the grounds of one of the country’s most prestigious universities, came to be known as “the Stanford murders.”

Ms. Perlov’s family was in the courtroom on Tuesday when the plea was announced.

“It has been 50 years this month since my older sister was taken from us,” Diane Perlov, her sister, said on Tuesday, The Mercury News of San Jose, Calif., reported. “Fifty years in which this monster has been free and living his life.”

Ms. Perlov was a protective older sister and guide for Diane Perlov, who is 14 months younger. After the killing, Diane Perlov said in an interview on Wednesday, it was her turn to become the protective one. She worked with police investigators for 45 years to help solve the murder of her sister.

“I did not want her to be voiceless and invisible,” she said.

Mr. Getreu, who the authorities now refer to as a serial killer, eluded officers for decades, mostly because DNA-evidence technology used to identify potential suspects was not as advanced as it became, said Michel Amaral, a deputy district attorney in the Santa Clara District Attorney’s Office.

A lawyer for Mr. Getreu, Matt Wilson, could not be immediately reached for comment on Wednesday night.

When Noe Cortez, a detective with the Santa Clara County Sheriff’s Office, re-examined the evidence in 2016, he noticed there was a significant chance to make a breakthrough, Mr. Amaral said.

The attack on her in the desolate woods had been brutal, Mr. Cortez determined from the evidence. Maybe she had scratched the assailant and fought back? Maybe underneath her fingernails, which had been stored and saved as evidence, was leftover DNA of the person who had killed her?

“She really fought hard for her life,” her sister said.

Mr. Cortez submitted the DNA collected from the fingernail clippings to a lab and tested his theory. He received a list of people associated with that DNA. One stuck out, Mr. Amaral said: Mr. Getreu, who, investigators learned, had been convicted in 1964 of raping and killing a 16-year-old girl in Germany. He had been tried as a minor, served a brief sentence and then traveled back to the United States, Mr. Amaral said.

In 2018, after the DNA was analyzed, Mr. Getreu became a prime suspect. That year, detectives were tracking Mr. Getreu and spotted him at a pharmacy in Union City, Calif., about 16 miles northeast of Stanford University, sipping from a Starbucks cup. After he threw the cup away, Mr. Amaral said, investigators retrieved it and used it to collect his DNA. Later, they found that the DNA collected from the fingernails matched that from the cup, Mr. Amaral added.

The authorities in Santa Clara County filed a murder charge in connection with Ms. Perlov’s case, and after sharing their findings with officials in San Mateo County, which was investigating the killing of Ms. Taylor, Mr. Getreu was charged in that killing as well.

The technology that solved the Perlov case and similar ones uses DNA databases, including that of ancestry records, to match evidence to a large network of people, said Claire Glynn, an associate professor and director of forensic genetic genealogy at the University of New Haven. The method has been increasingly used to help solve violent crimes, including in high-profile cases such as the Golden State Killer’s.

“This case is nearly 50 years old,” she said. “The ability to almost go backwards in time and to resolve historical cases and cold cases such as this, to get justice after 50 years, is just an astonishing thing.”

Ms. Perlov’s family thinks that she was in the woods on Feb. 13, 1973, the day the authorities believe she was killed, to photograph nature and commission a watercolor painting as a birthday gift for her mother.

Leslie Perlov, a Stanford history alumna interested in poetry and travel, had completed high school and college in three years each, said Diane Perlov, who would accompany her older sister to Vietnam War protests. Her family received Ms. Perlov’s acceptance letter to the University of Pennsylvania’s law school just after her death, her sister said. Her lifelong goal was to become the first female president of the United States.

“She was brilliant,” she said.