Explainers

What Is Investigative Genetic Genealogy, and How Does It Solve Cold Cases?

Investigative genetic genealogy (IGG) is a technique that takes crime-scene DNA, compares it to public consumer DNA databases to find distant relatives of an unknown suspect, and then uses traditional genealogy and family trees to narrow those relatives down to a single named person. It does not find the suspect directly; it finds the suspect’s family and works inward.

The Short Version

IGG combines forensic DNA analysis with old-fashioned genealogical research — building family trees backward from cousin matches.
It relies on public or opt-in consumer databases (notably GEDmatch and FamilyTreeDNA), not the police-only CODIS system.
It produces an investigative lead, not proof. A suspect is still confirmed by a fresh, direct DNA comparison before arrest.
The 2018 arrest of the Golden State Killer made the technique famous and helped open the door to an estimated 150-plus cold cases.
It works on both unidentified perpetrators and unidentified victims (John and Jane Does).

In Depth

How is IGG different from a normal DNA match?

Traditional forensic DNA matching runs a crime-scene profile against CODIS, the FBI-administered law-enforcement database of known offenders and arrestees. CODIS only works if the perpetrator’s own profile is already in the system. When there is no direct hit, the trail often goes cold.

IGG takes a different route. Instead of looking for a 100 percent match to the suspect, analysts look for partial matches — the kind of shared DNA you have with cousins, second cousins, and more distant relatives. Those relatives may have voluntarily uploaded their DNA to a consumer genealogy site for reasons that have nothing to do with crime. A handful of distant matches, combined with genealogical detective work, can triangulate toward one person.

What are the actual steps?

Build a dense DNA profile. Standard forensic kits read about 20 markers. IGG requires a much richer profile — often hundreds of thousands of genetic markers — typically produced through advanced sequencing by specialized labs.
Upload to a compatible database. The profile is searched against databases that permit law-enforcement use, principally GEDmatch and FamilyTreeDNA. Ancestry and 23andMe do not allow this kind of access.
Find relatives and build family trees. A genealogist identifies matching relatives and constructs extensive family trees, working forward and backward through census records, obituaries, and public documents.
Narrow the candidates. Investigators filter the family tree by age, sex, geography, and opportunity to isolate likely suspects.
Confirm with a direct sample. Police collect the suspect’s own DNA — often from a discarded item — and run a conventional, direct comparison to the crime-scene evidence before any arrest.

Is IGG reliable, and is it regulated?

IGG is best understood as a lead-generating tool, not a verdict. The genealogy narrows the field; the case is built on a direct DNA confirmation plus conventional evidence. That confirmation step is essential, because a family tree alone cannot prove which relative committed a crime.

The technique is only lightly and unevenly regulated. The U.S. Department of Justice has an interim policy for federally funded cases, but rules vary by state and agency, and privacy advocates have raised concerns about searching databases full of people who never consented to a criminal investigation. (Regulation is still developing and differs by jurisdiction — treat specific legal limits as state-dependent.)

Why does it work even when the suspect never tested their DNA?

This is the counterintuitive heart of IGG: a suspect does not have to be in any database. Because we each share measurable DNA with hundreds of relatives, it is often enough for a few cousins to have uploaded their data. Researchers have estimated that once a consumer database reaches a critical mass, a large share of a population can be identified through relatives alone — which is exactly why a never-tested offender can still be traced.

Seen in These Cases

The Golden State Killer: How Genetic Genealogy Caught Joseph DeAngelo — the case that put IGG on the map. Investigators uploaded the killer’s decades-old DNA to GEDmatch, built family trees from distant matches, narrowed to DeAngelo, then confirmed with DNA from a car door handle.
The Murder of Rachel Morin: How Her Killer Was Found — after CODIS failed to name a suspect, an FBI Investigative Genetic Genealogy team, working from a profile built by the lab Othram, traced relatives, built a family tree, and identified Victor Martinez-Hernandez, later convicted and sentenced to life without parole.

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