It was the case that haunted a small city in Alabama for nearly two decades: Two 17-year-old girls disappeared on the way to a birthday party and were later found shot to death in the trunk of a car.

There was evidence that one of the girls had been sexually assaulted, and investigators collected DNA from the crime scene. The police chased down hundreds of leads over the years, but could not identify a suspect.

Until this month.

On Monday, the Ozark Police Department announced that thanks to the same investigative technique that led to an arrest in the Golden State Killer case, a suspect was in custody. This is at least the fourth case in five days that has been solved using genetic genealogy, as the investigative technique is known. The arrest comes amid dueling efforts to expand and to ban this approach to identifying DNA left at crime scenes.

Chief Marlos Walker of the Police Department said that identifying the crime scene DNA after all these years brought him relief and also surprise: The suspect, Coley McCraney, 45, was a high school classmate. The two even played basketball together, he said.

“I had to sit in my chair for three hours,” he said in a news conference.

The district attorney said he planned to seek the death penalty against Mr. McCraney, who was arrested Friday and charged with capital murder and rape in the deaths of Tracie Hawlett and J.B. Beasley. The pair was found in Ms. Beasley’s car on the side of a road on July 3, 1999. They had both been shot in the head.

Ms. Hawlett’s mother, Carol Roberts, told The Associated Press that she went numb when she heard an arrest had been made.

“He didn’t have the right to do that,” Ms. Roberts said. “I just want to know why.”

Chief Walker said he decided to try genetic genealogy shortly after the highly publicized announcement in April that Joseph James DeAngelo, a former police officer, had been charged with the crimes associated with the notorious Golden State Killer.

“It raised my awareness as something we needed to check into,” he said in an interview.

The murder of the two teenagers was the obvious case to apply it toward, he said. A double murder is rare in Ozark, a city of around 15,000 that is about 80 miles south of Montgomery.

“That’s the only case of that nature around here,” Chief Walker said. “This is the crime that shook everyone; not just our city but even wider than that. It’s one of the biggest cases in the state of Alabama.”

Chief Walker hired Parabon, a forensic consulting firm specializing in genetic genealogy, to assist with the case. The first few steps involved in identifying DNA this way are typically the same.

A genetic profile is uploaded to GEDMatch, a genealogy database popular with family history researchers — and more recently, law enforcement. Then the team of genetic genealogists hopes for a close match, ideally something in the third cousin range. In this case, the relatives were more distant, said CeCe Moore, who leads Parabon’s genetic genealogy team.

“When I looked at this case initially I didn’t think it would be an easy solve,” Ms. Moore said. Still, her team was able to build out a family tree, connecting the closer matches to a common ancestor and then filling in the branches with an array of publicly available data.

In the end, it was a lucky coincidence that altered the direction of the investigation. Though the Parabon team was not able to create a probable suspect list, the family tree hinted at several possible surnames. When Chief Walker looked at the list, one stood out: McCraney.

“I recognized that last name,” he said, because he had had a high school classmate with that surname.

There was a solid chance that the McCraney he knew could be related to the suspect, and that his DNA would help direct the family tree building. Mr. McCraney agreed to provide DNA, according to his defense lawyer. When investigators compared his DNA to the DNA from the crime scene, it appeared to be an exact match, Chief Walker said.

“Like most people, I was surprised,” he said. Mr. McCraney did not have the type of record or reputation to suggest he had been involved in this type of crime, Chief Walker said.

“But I’ve also been doing this long enough to know that DNA is a solid thing to believe in,” he said. “I didn’t have any reservations when we got a match that this was him.”

Asked why Mr. McCraney agreed to provide DNA, David Harrison, his lawyer, responded: “Because he’s not guilty. And in the trial all these facts will come out.”

His client is an ordained bishop who has been driving a truck for the past 20 years, he said.

“Millions of Americans died for the right to be innocent until proven guilty,” Mr. Harrison said. It concerns him, he said, that once people hear the word “DNA,” they often assume the person must be guilty. He added that he would be requesting a change of venue for the trial.

The announcement from the Ozark police comes on the heels of three other cases that were solved using genetic genealogy in the past five days, and more than 40 since April, according to Ms. Moore.

Questions around the legality of this new practice remain unresolved. In January, Charles Sydnor, a Maryland House delegate, introduced a bill that would prevent law enforcement from identifying suspects’ DNA by using relatives in genealogy databases. The bill died within weeks, but it fueled discussion about whether the practice violates people’s privacy. Others have raised concerns about the excessive weight placed on genetic evidence, when there are hundreds of reasons a match may not be an indicator of guilt.

Defenders of the technique say law enforcement is limiting its searches to genealogy databases for which users have explicitly consented to assisting with murder and sexual assault investigations.

“It’s just a wonderful tool,” Chief Walker said. “I think more agencies will be using it within weeks and months down the road.”