Forensic genetic genealogy is an investigative technique that combines DNA analysis with traditional family tree research to identify unknown suspects or victims in criminal cases. It works by uploading a DNA profile from a crime scene to public genealogy databases, finding distant relatives of the unknown person, and then building out family trees until investigators can narrow down a specific individual. Since its first high-profile use in 2018 to identify the Golden State Killer, the technique has been used to solve over 1,000 cases.
Why Traditional DNA Databases Fall Short
Standard forensic DNA testing analyzes short tandem repeats, which are small repeating segments of DNA that vary from person to person. Crime labs compare these patterns against profiles stored in CODIS, the FBI’s national DNA database. If the suspect’s DNA is already in the system from a prior arrest or conviction, there’s a direct match. But if the person has never had their DNA collected by law enforcement, the search comes back empty.
Forensic genetic genealogy exists specifically for these dead ends. It is only attempted after all other techniques, including a CODIS search, have been exhausted. The method uses a different type of DNA analysis altogether, one that reads hundreds of thousands of single-nucleotide polymorphisms (SNPs) scattered across the genome. While STR profiles are useful for identifying a single person, SNP profiles reveal patterns of inheritance that can connect people across many generations, making it possible to find even distant cousins.
How the Process Works, Step by Step
The workflow starts with a biological sample from a crime scene: blood, saliva, skin cells, or other tissue. DNA is extracted and first run through standard STR analysis for a CODIS search. When that produces no match, investigators move to the genealogy approach.
The sample undergoes extensive SNP analysis, producing a detailed genetic profile that gets formatted for upload to a third-party genealogy database. The most commonly used platform is GEDmatch, a public database where people voluntarily upload DNA data they’ve received from consumer testing companies. The upload generates a list of potential genetic relatives, ranked by how much DNA they share with the crime scene sample. That shared amount is measured in centimorgans, a unit that reflects how closely two people are related. A parent-child pair shares around 3,400 centimorgans; a third cousin might share only 50 to 100.
From that list of matches, a genealogist (either working in-house for law enforcement or contracted) begins building family trees. Using the closest matches as starting points, they trace lineage backward through public records like birth certificates, marriage records, census data, and church records until they find the most recent common ancestor shared by the matching individuals and the unknown person. Then they build the tree forward again, generation by generation, identifying all living descendants of that ancestor.
The critical step is triangulation: when family lines from multiple matches converge on a single family unit, investigators examine that family’s members against the facts of the case. They look at sex, age, geographic location at the time of the crime, and any other circumstantial connections. This typically narrows the pool to one or a few individuals.
Confirming a Suspect
Forensic genetic genealogy does not produce a definitive identification on its own. It generates leads. Once investigators have a suspect, they still need to confirm the match through conventional DNA testing. This often involves collecting what’s called “abandoned” DNA, genetic material a person leaves behind in public, such as on a discarded coffee cup, a cigarette butt, or a utensil at a restaurant. That sample is then tested using standard STR analysis and compared directly to the crime scene evidence.
Courts have generally allowed this practice. In one notable case, State v. Carbo, the court ruled that the defendant had no expectation of privacy in DNA left behind at a crime scene, meaning police did not need a warrant to have it analyzed. The court did not, however, address the broader question of whether the deeply personal information contained in a full genetic profile carries its own privacy protections. That legal question remains unresolved.
DNA Quality Requirements
Not every crime scene sample works for this technique. SNP microarray testing, the most common method, typically requires around 200 nanograms of genomic DNA in reasonably good condition. Whole genome sequencing can work with roughly 100 nanograms, and newer specialized kits designed for forensic use have demonstrated results with as little as 100 picograms, a quantity about a thousand times smaller.
Degraded or contaminated samples pose a real challenge. When DNA breaks down over time, the SNP profile may contain gaps, producing fewer and weaker matches. The minimum threshold for a reliable segment match is generally accepted at 7 centimorgans. Below that, false positives become a serious problem. One study found that more than 67% of matches in the 2 to 4 centimorgan range were false positives. For cold cases involving decades-old evidence, DNA quality can be the difference between a viable investigation and a dead end.
Which Databases Are Used
Law enforcement does not have access to the major consumer testing companies’ databases (like those of 23andMe or AncestryDNA) for criminal investigations. Instead, investigators rely on platforms where users have voluntarily made their data available. GEDmatch is the primary one. Since May 2019, GEDmatch has required users to actively opt in before their profiles can be compared against law enforcement uploads related to violent crimes.
Users on GEDmatch can choose from four privacy tiers. “Public Opt-in” allows their DNA to be matched against law enforcement submissions for violent crime investigations. “Public Opt-out” keeps their data available for personal genealogy research and identification of human remains, but blocks violent crime searches. “Personal Research” hides the kit from other users’ match lists entirely, while “Private” removes the kit from all comparisons. Users can change their setting at any time. FamilyTreeDNA is another platform that has cooperated with law enforcement, though it also requires user consent.
Legal Guardrails
Federal policy sets a baseline. The Department of Justice’s interim policy requires that forensic genetic genealogy be used only after all other investigative techniques have failed, including a CODIS search. The policy limits its use to violent crimes and unidentified remains cases.
Some states have gone further. Montana passed legislation requiring law enforcement to obtain a search warrant based on probable cause before searching any consumer DNA database or conducting familial DNA searches. Maryland has enacted similar restrictions. These laws reflect growing concern that the technique, while powerful, touches the genetic privacy of thousands of people who are not suspects, since every database search inevitably reveals information about the relatives of the person being sought.
Scale and Impact
The growth of forensic genetic genealogy has been rapid. In 2019, roughly 78 cases had been solved using the technique. By 2024, that number exceeded 1,000. Many of these are cold cases that sat unsolved for decades, including sexual assaults, homicides, and unidentified remains cases where conventional methods had been completely exhausted.
The technique’s effectiveness depends heavily on the size and diversity of the databases being searched. Because most users of consumer DNA testing services are of European descent, cases involving suspects from other ethnic backgrounds produce fewer and more distant matches, making tree-building significantly harder. As database participation grows and diversifies, the investigative reach of the method expands. Even a third or fourth cousin match, when combined with skilled genealogy work and public records research, can be enough to identify someone whose DNA was left at a crime scene decades ago.