On a September afternoon in 2000, an FBI forensics analyst stood in her lab in Quantico, Virginia, staring into a microscope in confusion. She was looking at two strands of hair that were evidence in a murder investigation. And something wasn’t right. She checked the label on the evidence bag the sample came from, then looked back into the microscope. She definitely wasn’t imagining it. The label was wrong, which meant that the detective who had packaged the hairs and sent them over for analysis – had not understood what he was looking at. And this wasn’t a small mistake. Because if the analyst was right about what the hairs really were – then the whole investigation she was working on was about to change.