Two years into the investigation of Allison Feldman’s murder in Scottsdale, Arizona, the police had run out of options. They had interviewed more than 500 people, cataloged dozens of pieces of evidence and canvassed every house within a half mile of Feldman’s low-slung ranch home, where the popular 31-year-old saleswoman had been sexually assaulted and beaten to death.

DNA found in Feldman’s dining room and on a beer bottle had been detectives’ best lead. But, despite repeated queries, America’s vast crime databases had failed to match the DNA to a suspect.

Stymied, Scottsdale police appealed to Arizona officials to widen the parameters of the DNA search in a way the state had never done before. The detectives wanted to probe for the next best thing to a direct DNA hit — a match to anyone closely related to the mystery intruder.

About a year later, and more than three years after the grisly 2015 murder in a city known for very little violent crime, Scottsdale police and Arizona law enforcement leaders announced on April 10 that their unusual gambit had paid off.

Using software that searches for “familial” DNA links, crime lab technicians found a near match with an Arizona prison inmate. Days later, Scottsdale police arrested the inmate’s brother, Ian L. Mitcham, and charged him with Feldman’s murder.

“It was like ‘Wow! It actually worked.’ I couldn’t believe we had finally gotten to this point,” said Sgt. Hugh Lockerby, Scottsdale’s top violent crimes investigator. “It’s a real success story.”

This combination of two mug shots shows on the left, Ian Mitcham, who was recently arrrested for murder and on the right, his brother Mark Mitcham, currently in prison for child molestation, whose DNA helped police find his brother. Scottsdale Police Department/Arizona Dept of Corrections

While other states have used familial DNA matching to solve cold cases, the Feldman case was the first time Arizona had solved a case using the technique. But it was quickly followed by an even bigger case in California, the blockbuster arrest this week of a former policeman whom California authorities accused of being the serial rapist and murderer known as the Golden State Killer.

California investigators who arrested Joseph James DeAngelo, 72, in the Sacramento suburb of Citrus Heights on Tuesday also used his relatives’ DNA to track him down.

In the California case, though, authorities targeted a genealogical database that the public uses to search for relatives and ancestors, rather than the criminal DNA databases used in Arizona — a sign that investigators are now going further than ever before to use DNA to track down suspects.

But while anti-crime advocates say these new high-tech DNA tracking techniques could resolve many unsolved rapes, murders and other crimes, they are not without opponents.

The operators of several DNA services, like 23AndMe and Ancestry.com, have already said they do not make their data available to law enforcement. And GEDmatch.com, the service detectives used in the Golden State Killer case, told members they could take down their DNA profiles if they are uncomfortable with the fact that police can perform searches on the site.

Those developments could slow the use of consumer DNA sites for police work, experts said.

The use of government DNA records to track perpetrators through their relatives, which is known as “familial DNA” testing, has a more established record. Familial DNA provided the crucial breakthrough in 2010 that helped Los Angeles police find the “Grim Sleeper,” the notorious serial assailant who killed at least nine women and one teenage girl on the south side of the city.

In 2017, it cracked the 41-year-old strangulation murder of the ex-wife of one of the Righteous Brothers, the 1960s pop stars. And it helped Utah investigators find the man who they said used a hammer in 2011 to kill a 69-year-old woman in her home not far from the Great Salt Lake.

This undated family photo shows Allison Feldman, left, with her parents Elayne and Harley Feldman. Courtesy Feldman Family

At the news conference announcing Mitcham’s arrest, Col. Frank Milstead, Arizona’s director of public safety, called familial DNA “probably one of the biggest advancements in our lifetimes” in crime fighting. He added, “With this technology, we can bring years of frustration to an end.”

Yet Arizona and California are among just 12 states that employ familial DNA in criminal cases. The practice remains so uncommon that experts aren’t sure how many detectives and prosecutors are even aware DNA can provide an indirect pathway to suspects.

Familial DNA as an investigative tool has also faced opposition from civil libertarians, who worry it will be used indiscriminately, casting unwelcome attention on the relatives of crime suspects and particularly on people of color, because of their disproportionate representation in those databases.

In many states, though, the debate over familial DNA hasn’t even begun — leaving crime fighting behind what the latest technology would allow.

How familial DNA testing works

Traditional DNA searches take the genetic “fingerprints” found at crime scenes and run them through vast databases, made up mostly of people previously arrested or convicted of crimes. Computer software seeks to match genetic markers, or loci. Analysts currently focus on 20 of these markers. If all of them match, they can tie a person to a crime scene.

Previously, when direct DNA matches could not be made — often because the owner of the DNA has never been arrested or convicted of a crime — police had reached a dead end. But more than a decade ago, British police developed a new way to search for anyone who might be closely related to whoever left DNA at a crime scene. “Familial” matching was born.

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These searches look for individuals who carry only some genetic markers in common with those found in crime scene DNA. A match on 10 of the 20 critical markers strongly suggests a person is the parent, child or sibling of the suspect. Analysts performing familial searches often provide detectives with a list of near-matches, topped by these “first-degree” relatives.

The Golden State Killer case showed how investigators are now willing to cast an even wider net — using publicly available DNA data to identify more distant relatives and then work their way back to a suspect.

Detectives in the California case took their crime scene DNA profile and entered it into an online genealogy database, GEDmatch.com, which helps users find genetic matches. The tactic originally produced a list of about 100 men, which detectives winnowed down using standard police work, sources told The Los Angeles Times.

In the Scottsdale case, the investigation was much more precisely targeted. State crime lab technicians reported that they had only one familial DNA “hit,” and it pointed to Mark Mitcham, 54, who had been convicted in the early 1990s of child molestation and sentenced to 40 years in prison.

Scottsdale detectives were told Mitcham was almost certainly a close relative of the person who had been inside Feldman’s home.

Armed with that information, Scottsdale detectives quickly turned to Mitcham’s closest relatives. His father was dead, and two brothers did not have criminal records.

But his youngest brother, Ian, had a handful of misdemeanor arrests, including a DUI in 2015. Blood had been drawn in that case, so Scottsdale police used it to obtain a DNA sample. They compared it to the DNA from Feldman’s home, and the two samples matched perfectly.