Liz Flatt drove to Austin, Texas, mostly out of desperation. She had tried talking with the police. She had tried working with a former F.B.I. profiler who ran a nonprofit dedicated to solving unsolved murders. She had been interviewed by journalists and at least one podcaster. She had been featured on a Netflix documentary series about a man who falsely confessed to hundreds of killings. Although she didn’t know it at the time, Flatt was at a crossroads in what she had taken to calling her journey, a path embarked on after a prayer-born decision five years earlier to try to find who killed her sister, Deborah Sue Williamson, or Debbie, in 1975. It was now 2021. She had come to Austin for a conference, CrimeCon, which formed around the same time that Flatt began her quest, at a moment now seen as an inflection point in the long history of true crime, a genre as old as storytelling but one that adapts quickly to new technologies, from the printing press to social media. Flatt met ...