DNA databases and family searching have privacy implications in many areas, not just crime-solving:

Millions of consumers use genetic data to gain insight into family roots or learn about health risks. The boom has also revealed information test takers never expected, such as the identities of biological parents in adoptions or partners involved in secret relationships.

DNA databases have drawn interest from outsiders too--drug companies eager to mine them for information, researchers studying population migration and law enforcement seeking leads to crime suspects.

"Taking a DNA test does not just tell a story about me. DNA tests inevitably reveal information about many other people too, without their consent," says Natalie Ram, an associate professor of law at the University of Maryland Francis King Carey School of Law, who studies genetic privacy. "Should genetic databases be allowed to make up the rules as they go along?"

I would think that these other unforeseen consequences are more serious privacy concerns than finding out that a second cousin you never met or knew about is a serial killer.

The FBI asked the president of one online database company for help in two cases: an unknown dead child and a rape case. He agreed to help.

The dead child's identity wasn't revealed through matching in the FamilyTreeDNA database. But the rape case did generate leads, according to Mr. Greenspan. He said he learned much later the suspect was the man police alleged was the Golden State Killer, who was arrested in April 2018, and has been charged with multiple crimes. Police suspect him of murders and rapes over the course of decades.

The announcement of the Golden State Killer's arrest electrified the public. It also drew attention to the notion genealogy databases could help solve crimes. The suspect's DNA file had been uploaded to an open database run by the genealogy website GEDmatch.

I asked the reporter, Ms. Marcus, by email about that last sentence. She confirmed that law enforcement uploaded a DNA file of the perpetrator developed from crime scene evidence. There was no identified suspect at the time. It was the resulting match that enabled law enforcement to zero in on a suspect.

The matches resulting from these searches are used only as leads for investigation. No one is arrested or charged on the match alone. Typically, once further investigation identifies a particular suspect, that person's DNA would be obtained for direct testing, either by scooping up discarded material or by getting a court order for testing.

For the moment, it's up to the companies to decide whether to allow law enforcement use of their databases at all, and if so whether it will be on a customer opt-in or opt-out basis. Personally, I see no reason to be more restrictive about law enforcement use than other collateral uses, such as use by drug companies. Finding serial rapists does an enormous amount of good, both in closure for past victims and in prevention of future crimes.