The resulting searches found nearly 1,000 more pairs that matched at nine or more loci.
"I can appreciate why the FBI is worried about this," said David Kaye, an expert on science and the law at Arizona State University and former member of a national committee that studied forensic DNA.
But "people's lives do ride on this evidence," he said. "It has got to be explained."
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Concerned about errors
From her first discovery in 2001, Troyer and her colleagues in the Arizona Department of Public Safety's Phoenix DNA lab were intrigued.
At the time, many states looked at only nine or fewer loci when searching for suspects. (States now commonly attempt to compare 13 loci, and they may be able to search for more in the future. But even now, in many cases, fewer than 13 loci are discernible from crime scene evidence because of contamination or because of degradation over time.)
Based on Troyer's results, she and her colleagues believed that a nine-locus match could point investigators to the wrong person.
"We felt it was interesting and just wanted people to understand it could happen," said Troyer, who initially declined to be interviewed, then cautiously discussed her findings by telephone, with her bosses on the line.
"If you're going to search at nine loci, you need to be aware of what it means," said Todd Griffith, director of the Phoenix lab. "It's not necessarily absolutely the guy."
Troyer made a simple poster for a national conference of DNA analysts. It showed photos of the white man and the younger black man next to their remarkably similar genetic profiles.
Some who saw the poster said they had seen similar matches in their own labs.
Bruce Budowle, an FBI scientist who specializes in forensic DNA, told colleagues of Troyer that such coincidental matches were to be expected.
Three years later, Bicka Barlow, a San Francisco defense attorney, came across a description of Troyer's poster on the Internet.
Its implications became clear as she prepared to defend a client accused of a 20-year-old rape and murder.
A database search had found a nine-locus match between his DNA profile and semen found in the victim's body. Based on FBI estimates, the prosecutor said the odds of a coincidental match were as remote as 1 in 108 trillion.
Recalling the Arizona discovery, Barlow wondered if there might be similar coincidental matches in California's database -- the world's third-largest, with 360,000 DNA profiles at the time. The attorney called Troyer in Phoenix to learn more.
