Carol Dodge’s daughter had been killed, and a man had been convicted. But years later, still fixated on the case, she was poring over 30 hours of interrogation videos when it hit her that he could not be guilty.

What she saw on tape was not at all what she had envisioned when she heard that he had confessed.

“He would ask him a question,” she said of the interrogator, “and he would answer it for him.”

The man, Christopher Tapp, was exonerated Wednesday in Idaho Falls, Idaho, in part because of Ms. Dodge’s persistence, and in part thanks to novel forensic techniques that pair DNA with genealogy databases.

It was the first time genetic genealogy, a revolutionary technique that identifies suspects by matching crime scene DNA to relatives, has been used to clear a convicted killer’s name.

“The system thought that I could be thrown away,” Mr. Tapp said in a statement released through the Innocence Project. “They were wrong. I want people to know that I’m innocent. I’ve said it for the past 22 years. Finally, the truth is being revealed.”

The case also offers a reminder that under intense pressure, people sometimes confess to crimes that they did not commit, said Vanessa Potkin, the director of post-conviction litigation at the Innocence Project.

Mr. Tapp at first adamantly denied that he had any idea who killed the victim. But throughout the course of a 30-hour interview process spread over many weeks, investigators would not accept his story.

Law enforcement officials psychologically manipulated him into confessing, then used the confession as proof of his guilt even when DNA pointed to a different perpetrator, Ms. Potkin said.

On June 13, 1996, 18-year-old Angie Dodge was raped and murdered in her apartment in Idaho Falls. About a year later, the crime was still unsolved, when a friend of Ms. Dodge’s was arrested in an unrelated rape that also involved a knife.

The police believed that this man could be the perpetrator in the Dodge case. Mr. Tapp was friends with him. The police called Mr. Tapp to find out more about his friend.

But at some point in that process, Mr. Tapp, 20 at the time, became a suspect himself.

“Police threatened him with the death penalty and told him that if he just told them what they wanted to hear, they would give him immunity,” Ms. Potkin said.

When he was arrested, he renounced his confession. DNA evidence, extracted from semen at the scene, was not a match to Mr. Tapp.

But it did not matter. “The confession trumps all other evidence in a case,” Ms. Potkin said. “There is a common belief that I would never confess to a crime I didn’t do.”

Nearly a third of the 367 exonerations since 1989 in which the Innocence Project has used DNA testing to clear someone involved false confessions. “Interrogations are not a quest for information,” Ms. Potkin said. “The purpose is to get an admission.”

In May 1998, Mr. Tapp was convicted of rape and murder. The prosecutors’ case rested on Mr. Tapp’s confession and the testimony of a woman who said she overheard Mr. Tapp mention the murder at a party. Decades later, the woman would tell Ms. Dodge and a local newspaper that under pressure from the police, she had lied on the stand.

The defense argued that the confession was false and that investigators were ignoring DNA evidence that pointed to a different perpetrator. Nonetheless, the jury found him guilty. The prosecutor sought the death penalty, but the judge sentenced him to life in prison.

Ms. Dodge was left unsettled. Because of the DNA, she believed there was another person at large who had been involved in the death of her daughter.

Hoping to find some clues, she asked to see the confession tapes. Alone in her living room, she cued up the first one and prepared to take notes. The audio recording was poor quality, so she had to stop and replay large chunks.

“I had to keep playing them over and over and over again,” she said. It took more than 70 hours, she said, to get through about half that length of tape.

At the time of the interviews, which took place over several weeks, the police were already aware that Mr. Tapp was not the source of semen at the scene. They pushed him to name his accomplice and promised him leniency if he delivered. Twice they fed him a name, and he concurred. But neither matched the DNA. Finally, Mr. Tapp offered another name: “Mike.”

When she heard that, Ms. Dodge was appalled. It seemed likely, she said, that he was “an imaginary person,” offered up to make the interrogation stop.

In 2014, she reached out to a false confession expert, who contacted the Innocence Project. Mr. Tapp’s lawyers filed post-conviction motions based on additional DNA testing. And in 2017, the prosecutor agreed to a deal. The court vacated Mr. Tapp’s rape conviction, but not his murder conviction. He was freed for time served.

Over the years, Ms. Dodge encouraged the police to try new ways to identify the DNA. The traditional method — looking for a match in Codis, the F.B.I.’s criminal DNA database — had failed.

Finally Ms. Dodge called CeCe Moore, a genealogist with a consulting firm called Parabon, for help. After creating a new DNA profile from a degraded sample, Ms. Moore identified several relatives in a genealogy database. By building out their family trees and looking to see how they intersected, she identified Brian Leigh Dripps, who had lived across the street from the victim at the time of the murder.

After investigators confirmed that the crime scene DNA was his, Mr. Dripps was arrested. During his interview with the police, he not only confessed, but explicitly stated that he had acted alone and did not know Mr. Tapp.

The police and the prosecutors did not block the effort this week to clear Mr. Tapp. On Wednesday, Daniel Clark, the Bonneville County prosecutor, submitted a motion asking the judge to dismiss all charges against Mr. Tapp. “There is no doubt that there are failings in the criminal justice system and I think this case is evident of that,” he told a packed courtroom. “Hindsight is 2020 and this case certainly speaks to that effect.”

Mr. Dripps, who is currently in the Bonneville County Jail, will have his next court date in August, to the relief of Ms. Dodge.

“This has literally shattered my family as if we were a piece of glass,” Ms. Dodge said. “There is no way to pick up the pieces.”