He was a church-going father of two, and for more than 30 years Dennis Rader eluded police in the Wichita area, killing 10 people and signing taunting letters with a self-styled monogram: BTK, for Bind Torture Kill. In the end, it was a DNA sample that tied BTK to his crimes. Not his own DNA. But his daughter's.

Investigators obtained a court order without the daughter's knowledge for a Pap smear specimen she had given five years earlier at a university medical clinic in Kansas. A DNA profile of the specimen almost perfectly matched the DNA evidence taken from several BTK crime scenes, leading detectives to conclude she was the child of the killer. That allowed police to secure an arrest warrant in February 2005 and end BTK's murderous career.

The BTK case was an early use of an emerging tool in law enforcement: analyzing the DNA of a suspect's relatives. In the BTK example, police had a suspect and were looking to tie him to the crime. But now, states are moving to conduct familial searches of criminal databases, looking for close-to-perfect matches with DNA from crime scenes. A partial match with a convicted criminal could implicate a brother or daughter or father of the convict. Such searches, advocates say, constitute a powerful law enforcement tool that, experts say, could increase by 40 percent the number of suspects identified through DNA.

As things stand in some states, lab analysts who discover a potential suspect in this way may not be permitted to share that information with investigators. Such a policy, said William Fitzpatrick, a New York state district attorney, "is insanity. It's disgraceful. If I've got something of scientific value that I can't share because of imaginary privacy concerns, it's crazy. That's how we solve crimes."

But the technique is arousing fierce objections from privacy advocates, who maintain that it turns family members into genetic informants without their knowledge or consent. They complain that it takes material collected for one purpose and uses it for another. And with the nation's DNA database disproportionately comprised of minority offenders, they say, it amounts to placing a class of Americans under greater scrutiny merely because their relatives have committed crimes.

"If practiced routinely, we would be subjecting hundreds of thousands of innocent people who happen to be relatives of individuals in the FBI database to lifelong genetic surveillance," said Tania Simoncelli, science adviser to the American Civil Liberties Union.

Nonetheless, California, which maintains the world's third-largest criminal DNA database with more than 1 million samples, will soon become the first state to adopt a protocol to allow for familial searches. Last week, Colorado performed a test run of familial search software on its criminal database. In Massachusetts, officials say they plan to develop a policy to allow familial searches.

The technique is being adopted as states and the federal government expand their databanks to include profiles of people who have been arrested but not convicted of certain crimes.

Only Maryland has expressly banned familial searching in a law adopted this month to expand its DNA database to include anyone charged with a violent crime. The FBI, which maintains the world's largest forensic DNA database with almost 6 million profiles, said it has so far refrained from adopting the technique because of concerns about constitutional challenges.

"The FBI would be more comfortable with congressional authorization to conduct familial searches," said Thomas Callaghan, head of the FBI's national DNA database.

However, he said, the bureau does occasionally find partial matches. When it does, under an interim policy, it allows states to follow up to see whether a relative is involved.

An advisory group to the FBI has proposed a final policy that goes further, recommending that partial matches be subjected to additional DNA testing and statistical analysis that would help investigators home in on relatives of people in the federal database.