Forensic Genealogy: How DNA Databases Solved Decades-Old Cold Cases

The Golden State Killer Breakthrough

On 24 April 2018, California police arrested Joseph James DeAngelo, a 72-year-old former police officer, for a series of murders, rapes, and burglaries committed across the state between 1974 and 1986. The case had been cold for decades. What changed was not new physical evidence, but a new investigative technique: forensic genealogy.

Investigators uploaded crime scene DNA to GEDmatch, a public genealogy database where users share genetic data to find relatives. The DNA did not match DeAngelo directly — he had never submitted his own sample. Instead, it matched distant cousins. By constructing family trees and eliminating suspects, investigators narrowed their search to DeAngelo. Surveillance confirmed the match; a discarded item yielded his DNA.

This arrest inaugurated a new era in cold case work. Within five years, forensic genealogy had been credited with solving over 200 cases in the United States alone.

How Forensic Genealogy Works

Forensic genealogy combines traditional DNA analysis with genealogical research. The process unfolds in stages:

DNA Upload. Crime scene DNA is processed to generate a genetic profile, then uploaded to consumer genealogy databases such as GEDmatch or FamilyTreeDNA. These platforms allow law enforcement uploads under specific policies.

Relative Matching. The database returns a list of genetic relatives — typically third or fourth cousins who share small DNA segments with the unknown perpetrator. A fourth cousin shares a great-great-great-grandparent; there may be hundreds of such matches.

Tree Construction. Genealogists build family trees backward from the matches, using birth records, marriage certificates, census data, and obituaries. The goal is to identify the common ancestors shared by the matches.

Descendancy Research. Once common ancestors are identified, genealogists trace forward through all their descendants, constructing a pool of potential suspects. This pool may contain dozens of individuals.

Winnowing. Investigators eliminate candidates using age, geography, and other case facts. If the crime occurred in Sacramento in 1978, a descendant born in 1985 or living in New York is excluded.

Confirmation. Police obtain a direct DNA sample from the remaining suspect — often from discarded trash — and compare it to crime scene DNA. If it matches, an arrest follows.

The technique is labour-intensive. A single case may require months of genealogical work and access to subscription genealogy services, vital records, and historical newspapers.

Notable Cases Solved

Sherri Rasmussen Murder (1986). Rasmussen, a Los Angeles nurse, was beaten and shot in her home. The case went cold despite DNA evidence. In 2008, detectives re-examined the case, but forensic genealogy was not yet available. It was old-fashioned detective work — and a 2009 DNA database hit on a police officer, Stephanie Lazarus — that solved this case. Lazarus, a former girlfriend of Rasmussen's husband, was convicted in 2012. While not a forensic genealogy case, it demonstrated the power of re-examining cold cases with modern techniques.

April Tinsley Murder (1988). Eight-year-old April Tinsley was abducted and murdered in Fort Wayne, Indiana. The killer taunted police for years, leaving notes and used condoms in public places. DNA was preserved, but yielded no database matches. In 2018, Parabon NanoLabs used forensic genealogy to identify John Miller, who lived blocks from where Tinsley's body was found. Miller confessed and died in prison in 2024.

Nikki Allan Murder (1992). Seven-year-old Nikki Allan was murdered in Sunderland, England. A local man was tried and acquitted in 1995. The case remained unsolved until 2021, when familial DNA searching identified David Boyd. Boyd was convicted in 2023. This case demonstrated forensic genealogy's expansion beyond the United States.

Jay Cook and Tanya Van Cuylenborg Murders (1987). The young couple was murdered during a trip to Seattle. Crime scene DNA was preserved but unmatched for decades. In 2018, Parabon used forensic genealogy to identify William Talbott II, who was convicted in 2019. This was one of the first cases solved publicly using the technique after the Golden State Killer arrest.

The Technology Behind the Method

Forensic genealogy relies on single nucleotide polymorphism (SNP) analysis rather than the short tandem repeat (STR) analysis used in criminal databases like CODIS. Consumer DNA tests examine hundreds of thousands of SNPs across the genome, providing far more data points than the 20 STR loci used in forensic databases.

This matters because distant relatives share small DNA segments. An STR profile is too coarse to detect third or fourth cousins. SNP analysis reveals these distant connections.

Once a genealogical lead is developed, investigators obtain a conventional STR profile from the suspect to confirm the match. Courts have consistently held that this two-stage process satisfies evidentiary standards.

Legal and Ethical Considerations

Forensic genealogy has sparked debate. Privacy advocates argue that individuals who upload DNA to find relatives do not consent to their data being used in criminal investigations. A person's genetic information can implicate family members who never agreed to participate.

In practice, most databases now require law enforcement to disclose their purpose. GEDmatch, after initial controversy, implemented an opt-in policy: users must explicitly allow law enforcement searches. FamilyTreeDNA permits law enforcement access by default.

Legal challenges to forensic genealogy have been rare and largely unsuccessful. Courts have held that individuals have no expectation of privacy in genetic information they voluntarily share with third parties. The technique does not violate Fourth Amendment protections against unreasonable search.

Racial and class disparities are a concern. Genealogy databases are disproportionately white and middle-class. A 2018 study found that forensic genealogy is most effective for suspects of European ancestry. This creates an uneven investigative landscape.

The Future of Cold Case Investigation

Forensic genealogy is now standard practice in many jurisdictions. The FBI, once sceptical, has trained agents in the technique. State and local agencies contract with private firms like Parabon NanoLabs, Bode Technology, and Othram to perform genealogical searches.

The technique is expanding beyond murder. In 2023, forensic genealogy identified a suspect in a 1996 sexual assault in Texas. Investigators are also using it to identify remains of unknown victims — so-called Doe cases.

Technological refinements continue. Whole genome sequencing can extract usable DNA from degraded samples that would have been unusable a decade ago. Investigative genetic genealogy software automates portions of tree-building, reducing the time required per case.

The technique's success has created a secondary effect: suspects are confessing. When confronted with genealogical evidence, many perpetrators — often elderly men who believed they had escaped justice — admit guilt. The psychological weight of being identified through family connections appears profound.

Limitations and Challenges

Forensic genealogy is not a panacea. It requires high-quality DNA samples. Degraded or mixed samples may not yield sufficient SNP data. It requires ancestry in the database's user population. A suspect with no relatives in genealogy databases cannot be identified this way.

It is expensive and slow. A single case may cost tens of thousands of dollars in laboratory and genealogical fees and require six to eighteen months of work. Agencies must prioritise cases carefully.

Finally, it is genealogically complex. Adoption, non-paternity events, and incomplete records can derail tree construction. Genealogists must navigate historical complexities with care.

Despite these limitations, forensic genealogy represents the most significant advance in cold case investigation since the development of DNA profiling itself. For families who have waited decades for answers, it offers something once unimaginable: resolution.

Conclusion

The intersection of consumer genetics and criminal investigation has rewritten the rules of cold case work. Techniques that were science fiction in the 1980s are now routine. The cases once filed away as unsolvable are being reopened.

Forensic genealogy does not replace traditional detective work — it amplifies it. Investigators still need to understand victimology, crime scene analysis, and interview technique. But when physical evidence exists and leads have dried up, genealogy offers a path forward.

The message to perpetrators is clear: time does not erase guilt. DNA does not forget. And family trees, once the province of hobbyists seeking ancestors, have become instruments of justice.