Investigators who used a genealogical website to find the ex-policeman they believe is a California serial killer and rapist call the technique ground-breaking.
But others say it raises troubling legal and privacy concerns for the millions of people who submit their DNA to such sites to discover their heritage.
There are no strong privacy laws to keep police from searching ancestry site databases, said Steve Mercer, the chief lawyer for the forensic division of the Maryland Office of the Public Defender.
"People who submit DNA for ancestors testing are unwittingly becoming genetic informants on their innocent family," Mr Mercer said, adding that they "have fewer privacy protections than convicted offenders whose DNA is contained in regulated databanks".
Joseph James DeAngelo, 72, was arrested on Tuesday after investigators matched crime-scene DNA with genetic material stored by a distant relative on an online site.
From there, they narrowed it down to the Sacramento-area grandfather using DNA obtained from material he had discarded, Sacramento County District Attorney Anne Marie Schubert said.
Authorities declined to name the online site. However, two of the largest, Ancestry.com and 23andMe, said they were not involved in the case.
DNA potentially may have played an earlier role in the case. It was just coming into use as a criminal investigative tool in 1986 when the predator variously known as the East Area Rapist and the Golden State Killer apparently ended his decade-long wave of attacks.
DeAngelo, a former police officer, probably would have known about the new method, experts said.
"He knew police techniques," said John Jay College of Criminal Justice professor Louis Schlesinger. "He was smart."
No one who knew DeAngelo over the decades connected him with the string of at least a dozen murders, 50 rapes and dozens of burglaries from 1976 to 1986 throughout the state.
After he was identified as the suspect, however, prosecutors rushed to charge him with eight killings.
Investigators searched DeAngelo's home on Thursday, looking for rings, earrings, dishes and other items that were taken from crime scenes as well as weapons.
Meanwhile, DeAngelo's neighbours, relatives and former acquaintances all say they had no inkling that he could be a serial killer.
He worked nearly three decades in a Sacramento-area supermarket warehouse as a truck mechanic, retiring last year. As a neighbour, he was known for taking meticulous care of his lawn in suburban Citrus Heights.
DeAngelo worked as a police officer in the farming town of Exeter, not far from Visalia, from 1973 to 1976.
DeAngelo was a "black sheep" who did not joke around with other officers, said Farrel Ward, 75, who served on the force with DeAngelo.
"I've been thinking, but there's no indication whatsoever that anything was wrong," he said. "How could you just go out and kill somebody and go back and go to work? I don't understand that."
Later, DeAngelo joined the Auburn Police Department outside of Sacramento but was fired in 1979 after he was caught shoplifting a hammer and dog repellent.
Investigators say they have linked DeAngelo to 11 killings that occurred after he was fired.