At dawn in late January 1998, two men entered the home of Betty Black in Farmers Branch, a suburb of Dallas, Texas. They killed her in an apparent burglary gone wrong. A few hours later, an eyewitness — Black’s neighbour — described what she had seen to police. She said that two white men with long hair had got out of a car and walked towards Black’s house in the early morning light. The neighbour, Jill Barganier, went to the police station the next day and identified Richard Childs, a white man with long hair, as the car’s driver. Childs would later confess to his involvement and serve 16 years in prison. She was convicted of killing her four children. Could a gene mutation set her free? Over the next week, the police homed in on 28-year-old Charles Don Flores as the second suspect. Flores had been seen with Childs on the morning of the murder, but he was a Latino man with short hair. On 4 February, Barganier was called to the police station.…