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.
Now, lawyers around the country are asking for searches of their own state databases.
Several scientists and legal experts want to test the accuracy of official statistics using the 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.
As a result, Thomas Callaghan, head of the FBI's CODIS unit, has dismissed Troyer's findings as "misleading" and "meaningless."
He urged authorities in several states to object to Arizona-style searches, advising them to tell courts that the probes could violate the privacy of convicted offenders, tie up crucial databases and even lead the FBI to expel offending states from CODIS -- a penalty that could cripple states' ability to solve crimes.
In one case, Callaghan advised state officials to raise the risk of expulsion with a judge but told the officials that expulsion was unlikely to actually happen, according to a record of the conversation filed in court.
In an interview with The Times, Callaghan denied any effort to mislead the court.
The FBI's arguments have persuaded courts in California and other states to block the searches. But in at least two states, judges overruled the objections.
