INFORMATION WANTS TO BE FREE:

The detectives in the Golden State Killer case uploaded the suspect's DNA sample. But they would have had to check a box online certifying that the DNA was their own or belonged to someone for whom they were legal guardians, or that they had "obtained authorization" to upload the sample.

"The purpose was to make these connections and to find these relatives," said Blaine Bettinger, a lawyer affiliated with GEDmatch. "It was not intended to be used by law enforcement to identify suspects of crimes."

But joining for that purpose does not technically violate site policy, he added.

Erin Murphy, a law professor at New York University and expert on DNA searches, said that using a fake identity might raise questions about the legality of the evidence.

The matches found in GEDmatch were to relatives of the suspect, not the suspect himself.

Since the site provides family trees, detectives also were able to look for relatives who might not have uploaded genetic data to the site themselves.

On GEDmatch, "it just happens they got lucky," said Dr. Ashley Hall, a forensics science expert at the University of Illinois in Chicago.

23andMe has more than 5 million customers, and Ancestry.com has 10 million. But the DNA in databases like these are relevant to tens of millions of others -- sisters, parents, children. A lot can be learned about a family simply by accessing one member's DNA.

"Suppose you are worried about genetic privacy," Ms. Murphy said. "If your sibling or parent or child engaged in this activity online, they are compromising your family for generations."

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