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In the summer of 2002, the FBI, the Baton Rouge Police Department, and several other agencies began a massive search for a serial killer suspected of murdering three women. Based on an FBI profile and an eyewitness report, they upended southern Louisiana looking for a white man who drives a white pickup, collecting DNA from more than 1,000 Caucasian males. They found nothing. Meanwhile, the killer struck again.

In March 2003, investigators turned to Tony Frudakis, a molecular biologist who said he could determine the suspect's race by analyzing his DNA. Uncertain about the science, the police asked Frudakis to take a blind test: They sent him DNA swabs from 20 people to see if he could identify their races. He nailed every one.

On a conference call a few weeks later, Frudakis reported his results on their killer. "Your guy could be African-American or Afro-Caribbean, but there is no chance that this is a Caucasian." There was a prolonged silence, followed by a flurry of questions. They all came down to this: Would Frudakis bet his life on his...