How reliable is DNA in identifying suspects?

State crime lab analyst Kathryn Troyer was running tests on Arizona’s DNA database when she stumbled across two felons with remarkably similar genetic profiles.

The men matched at nine of the 13 locations on chromosomes, or loci, commonly used to distinguish people.

The FBI estimated the odds of unrelated people sharing those genetic markers to be as remote as 1 in 113 billion. But the mug shots of the two felons suggested that they were not related: One was black, the other white.

In the years after her 2001 discovery, Troyer found dozens of similar matches — each seeming to defy impossible odds.

As word spread, these findings by a little-known lab worker raised questions about the accuracy of the FBI’s DNA statistics and ignited a legal fight over whether the nation’s genetic databases ought to be opened to wider scrutiny.

The FBI laboratory, which administers the national DNA database system, tried to stop distribution of Troyer’s results and began an aggressive behind-the-scenes campaign to block similar searches elsewhere, even those ordered by courts, a Times investigation found.

At stake is the credibility of the compelling odds often cited in DNA cases, which can suggest an all but certain link between a suspect and a crime scene.

When DNA from such clues as blood or skin cells matches a suspect’s genetic profile, it can seal his fate with a jury, even in the absence of other evidence. As questions arise about the reliability of ballistic, bite-mark and even fingerprint analysis, genetic evidence has emerged as the forensic gold standard, often portrayed in courtrooms as unassailable.

But DNA “matches” are not always what they appear to be. Although a person’s genetic makeup is unique, his genetic profile — just a tiny sliver of the full genome — may not be. Siblings often share genetic markers at several locations, and even unrelated people can share some by coincidence.

No one knows precisely how rare DNA profiles are. The odds presented in court are the FBI’s best estimates.

The Arizona search was, in effect, the first test of those estimates in a large state database, and the results were surprising, even to some experts.

Defense attorneys seized on the Arizona discoveries as evidence that genetic profiles match more often than the official statistics imply — and are far from unique, as the FBI has sometimes suggested.

Lawyers seek searches

Now, lawyers around the country are asking for searches of their own state databases.

Several scientists and legal experts as well want to test the accuracy of official statistics using the nearly 6 million profiles in CODIS, the national system that includes most state and local databases.

“DNA is terrific and nobody doubts it, but because it is so powerful, any chinks in its armor ought to be made as salient and clear as possible so jurors will not be overwhelmed by the seeming certainty of it,” said David Faigman, a professor at UC Hastings College of the Law, who specializes in scientific evidence.

FBI officials argue that, under their interpretation of federal law, use of CODIS is limited to criminal justice agencies. In their view, defense attorneys are allowed access to information about their specific cases, not the databases in general.

Bureau officials say critics have exaggerated or misunderstood the implications of Troyer’s discoveries.

Indeed, experts generally agree that most — but not all — of the Arizona matches were to be expected statistically because of the unusual way Troyer searched for them.

In a typical criminal case, investigators look for matches to a specific profile. But the Arizona search looked for any matches among all the thousands of profiles in the database, greatly increasing the odds of finding them.