On a rainy Thursday in January 1982, Anne Pham was getting ready for kindergarten at her family’s home in Seaside, Calif. Having developed an independent streak as one of 10 siblings, the 5-year-old successfully pleaded with her mother and an older brother to let her walk the two blocks to school by herself.

But nobody at a busy grocery store along her route saw Anne. Nobody saw her at school, either.

Not until dinnertime did her large family notice her absence. For two days, there was no sign of her. Then, in some bushes by a road less than two miles away, her body was discovered by accident. She had been sodomized and smothered to death.

The Seaside Police Department and the Federal Bureau of Investigation never developed a suspect or even a lead — until this year.

With a recent resolve to examine unsolved cases, a mysterious strand of hair and the help of genetic genealogy — which has been used to crack unsolved cases across the country in recent years — the authorities in Monterey County were able to identify Robert J. Lanoue, 70, of Reno, Nev., as a suspect and charge him with first-degree murder in Anne’s killing.

He is currently being held in Nevada pending extradition to California, said Jeannine P. Pacioni, the Monterey County district attorney.

Mr. Lanoue was 29 at the time of the Jan. 21, 1982, murder and lived just one block away. He was a staff sergeant at Fort Ord, then an Army base near Seaside.

In 1998, Mr. Lanoue was convicted of possessing and producing child pornography and committing a lewd act with a child under 16 in Nevada, where he is a registered sex offender. He spent about two decades in prison, Nick Borges, the chief of the Seaside Police Department, said in an interview.

Public records did not list a lawyer for Mr. Lanoue and Matthew L’Heureux, a deputy district attorney in Monterey County, said he did not know whether Mr. Lanoue had a lawyer.

Chief Borges said that he was not aware of prior criminal accusations against Mr. Lanoue but added that he was “highly concerned.”

“I don’t think you wake up one day and think, ‘I’m going to kill a child, I’m going to rape a child, and then I’m going to move on and be a wonderful citizen,’” Chief Borges said.

Anne’s family, like so many others, had fled Vietnam by boat in the aftermath of the war and wound up in the United States, Chief Borges said.

“It was paradise compared to what this family had seen in the Vietnam War,” Chief Borges said of their impression of the U.S. “They never thought coming here, to this paradise, would be the reason their daughter would pass.”

According to a 1982 article in The Monterey Peninsula Herald, Anne was the first member of her family to be born in the U.S., in 1976. She was named after a church that sponsored the family, St. Anne’s Catholic Church in Beaumont, Texas. Anne dressed herself and combed her own hair every morning and liked going to school, The Herald said.

Chief Borges came across the cold case in 2009 while going through a box filled with material from several unsolved cases. He was “disgusted” by the condition of the evidence in Anne’s case and vowed to do something about it, he said.

Two years ago, a detective looking into cold cases for the district attorney asked him about cases that were “extremely complex and sensitive,” Chief Borges recalled. “Without hesitation, I said, ‘Annie Pham.’”

The case was broken open thanks to a single piece of evidence left over from the original investigation — a centimeter-long rootless hair. It was unclear initially whether the hair belonged to a male or female.

Kelly Harkins, the chief executive of Astrea Forensics, said in an interview that her lab was able to extract DNA from the hair and sequence it. CeCe Moore, a genetic genealogist, said that she was then able to establish a “genetic network” in which the same unusual surname kept recurring — Lanoue.

“We got fortunate in this case that the key surname identified through the genetic network that I was able to create ended up being the surname of the suspect,” she said. A detective discovered that Robert Lanoue had been a neighbor of Anne’s, and further investigation revealed him to be source of the hair, Chief Borges said, though he declined to specify how exactly the authorities had determined that.

In March, Chief Borges ordered a large cardboard cutout of Anne, which he installed in the lobby of his office.

On Thursday, the day the charges were announced, Chief Borges called his team and said, “We’re going to take Annie to school today.”

He carefully fit his picture into his car, not wanting to put it in the trunk, since he said that is where Anne likely was when she was killed. He and a group of detectives drove to Highland Elementary School, Anne’s old school.

“She didn’t make it to school that day,’’ Chief Borges said, “but the day this case was filed, the day a suspect was arrested, she did.”