The short version
Investigative genetic genealogy (IGG) finds an unknown person through their relatives. A lab turns crime scene DNA into a genetic profile. That profile is compared against the people in genetic genealogy databases. The matches are distant relatives, usually cousins. Genealogists build those cousins' family trees back to shared ancestors, then forward again until the trees point to a small set of people who could be the source of the DNA. Investigators then confirm or rule out each one with a direct DNA comparison.
The method is also called forensic genetic genealogy (FGG). It works on violent crimes with an unknown offender and on unidentified human remains.
- 01Choose the caseViolent crime or unidentified remains. CODIS found no match.
- 02Develop a SNP profileA lab reads hundreds of thousands of markers from the evidence.
- 03Search the databasesUpload the profile and get a list of DNA relatives.
- 04Read the matchesEstimate how each relative is related.Indago automates
- 05Build the family treesTrace clusters back to common ancestors.Indago automates
- 06Narrow to candidatesFollow descendants to where the lines meet.The lead
- 07Confirm by DNACompare the candidate’s DNA directly with the evidence.
- 08Prepare for courtDocument the work for your prosecutor.
How it differs from CODIS
CODIS compares about two dozen short tandem repeat (STR) markers. It finds a person only if that person, or in some states a close relative, is already in the database. Most people aren't.
IGG reads hundreds of thousands of single nucleotide polymorphisms (SNPs), the same kind of data consumer DNA tests produce. That density is what lets it detect distant relatives like third and fourth cousins. Millions of people have tested with consumer companies, so even an offender who has never tested usually has relatives who have.
20 markers
Finds a person only if they, or in some states a close relative, are already in the database.
Hundreds of thousands of markers
Dense enough to find relatives as distant as third and fourth cousins, who have usually tested.
The two work together. A CODIS search comes first, and IGG is for cases where it found nothing.
The process, step by step
1. Choose the case
Not every case qualifies. Under the Department of Justice's interim policy, IGG is for homicides, sexual assaults and unidentified human remains, and for other violent crimes when a prosecutor approves. A CODIS search has to come back without a match first. Many states add their own rules. Check your agency's policy and talk to your prosecutor before you start.
2. Develop a SNP profile
A lab extracts DNA from the evidence and generates a SNP profile. Degraded or mixed samples, old evidence and small quantities all make this harder. Whole-genome sequencing can recover usable data from samples that older methods couldn't. Lab work is usually the largest single cost of a case.
3. Search the genealogy databases
The profile is uploaded to genetic genealogy databases. Each search returns a list of people who share DNA with the unknown person.
4. Read the matches
Each match comes with the amount of DNA shared with the unknown person, measured in centimorgans (cM). More shared DNA means a closer relationship. The ranges overlap, though. A given amount of shared DNA can fit several different relationships, so every match is a set of possibilities, not a single answer. Ages, where known, help narrow it down.
Matches that share DNA with each other usually descend from the same ancestral couple. Grouping them into clusters is how genealogists tell the unknown person's different family lines apart.
5. Build the family trees
For each cluster, genealogists research the matches' ancestry using public records: birth, marriage and death records, census entries, obituaries and newspapers. The goal is to find the common ancestors each cluster shares, then trace every line of their descendants forward to the present.
This is the slowest part of the work. A single case can mean thousands of people across several trees, and it can take weeks or months by hand.
6. Narrow to candidates
Where two family lines meet, often through a marriage, the unknown person is likely a descendant of that couple. Investigators then use what's known about the case to narrow the descendants: sex, approximate age, where the person lived at the time, and physical description. The result is a short list of candidates, sometimes a single name.
- 1Matches that share DNA with each other form clusters. Each cluster is one family line.
- 2Records research traces each cluster back to the couple its members descend from.
- 3Descendants of both couples meet in a marriage. Their children are the candidates.
7. Confirm with a direct DNA comparison
A genealogy result is an investigative lead. It's never enough on its own to charge someone. Investigators confirm it by comparing the candidate's DNA directly with the evidence, usually through a reference sample collected lawfully. Only that comparison identifies the source.
8. Prepare for court
The confirmation is what goes to court. Defense counsel will still ask how the candidate was found, so keep a clear record of the genealogy work and follow your prosecutor's guidance on discovery.
The rules that govern it
- The DOJ interim policy. In effect since November 2019 for federal cases and DOJ-funded work. It limits IGG to homicides, sexual assaults and unidentified remains, with other violent crimes allowed when a prosecutor approves. It requires a CODIS search first, consultation with a prosecutor, and confirmation by direct DNA comparison before an arrest.
- State law. Maryland and Montana regulate the practice by statute, and other states have followed. Maryland, for example, requires judicial authorization before a search.
- Database terms. Each genetic genealogy database sets its own terms for how its data may be used.
- Your agency's policy. Many agencies add their own approvals, case criteria and record-keeping requirements.
Where it started
The Golden State Killer had evaded identification for decades. In 2018, a small team used genetic genealogy to identify him. Steve Kramer co-led that team. With Stephen Busch, he then built the FBI's genetic genealogy program. Since then it has helped solve hundreds of cold cases and give names back to people found as unidentified remains.
Where Indago fits
Steps 4 to 6 are where cases stall. Indago automates the slowest of them. It brings in authorized DNA matches, estimates how each match is related, builds the family trees and checks the people in them against connected records. Your investigators review every step and decide what becomes a lead. Read AI in investigations, responsibly for how that review works, or see the platform.