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.