How Police Solve Cold Cases With DNA
In this post
- What Makes a Case Cold, and Why DNA Revives It
- From Crime Scene Sample to DNA Profile
- Criminal Databases: The First Comparison
- Forensic Genealogy: Finding Family, Then a Suspect
- Retesting Old Evidence With New Technology
- Building a Case Beyond the DNA Match
- Privacy, Consent, and the Rules Still Being Written
- My Take:
A cold case is a criminal investigation that has stalled, not one that has been forgotten. Files remain, evidence remains, and families remain, waiting for a development that may come years or decades later. In recent years, DNA has provided that development in cases once thought unsolvable, including murders and assaults from the 1970s, 1980s, and 1990s. A sample too small, degraded, or complex for older methods can sometimes be analysed with modern techniques, and a profile that matched no one in a criminal database can sometimes be linked to a family through genealogy.
This article explains how that process works, step by step, without overstating what DNA can do. DNA does not identify a criminal by itself in every case, it does not work on every old sample, and its use raises privacy questions that police and courts must handle carefully. Understanding the method, its limits, and its safeguards helps readers judge news reports about solved cold cases more accurately, and helps families understand why some cases move forward while others remain open.
What Makes a Case Cold, and Why DNA Revives It
A case becomes cold when leads are exhausted, witnesses cannot be found or cannot recall details, and no suspect can be charged with the evidence available. That does not mean investigators stop caring. Cold-case units periodically review files, check whether new technology applies, and look for evidence that was preserved but never fully tested. Sometimes a case revives because a witness comes forward, sometimes because a suspect is arrested for another offence, and increasingly because a DNA sample can now be read in a way that was impossible when the crime happened.
Older DNA methods needed relatively large, good-quality samples and produced profiles that could only be compared with known suspects or criminal databases. If the offender had never been arrested, or if the sample was mixed with other DNA, degraded by time, heat, or moisture, the result might have been inconclusive. Modern methods can work with smaller amounts, separate mixed samples more effectively, and read more markers, which are the points of variation used to compare profiles. That does not make every old sample usable, but it expands the set of cases worth retesting.
Preservation is the hidden hero of cold-case DNA work. Evidence stored in paper bags, kept dry and cool, and handled with documented chain of custody has a far better chance of yielding DNA decades later than evidence stored poorly. Many solved cold cases depend on decisions made by officers and laboratory staff long ago, who kept clothing, swabs, and other items even when no test existed to use them. When a laboratory today opens a decades-old envelope, it is benefiting from that foresight, and also from the discipline of not discarding evidence simply because a case went cold.
From Crime Scene Sample to DNA Profile
The process begins with selecting the right item and the right part of it. A forensic scientist examines case notes, photographs, and the item itself, looking for areas most likely to hold the offender’s DNA, such as a torn garment, a weapon handle, or a swab taken from a victim. The scientist must balance the desire for a result against the risk of consuming the sample. If only a tiny amount remains, a failed test could destroy the last chance, so laboratories often start with non-destructive examination and preserve part of the sample for future methods.
Extraction separates DNA from cells, and amplification copies the relevant markers many times so they can be measured. The result is a profile: a set of numbers representing variation at specific locations in the genome. That profile can be compared with a suspect’s profile, with a victim’s profile to exclude their DNA, and with database profiles. A match does not mean the profiles are identical in every respect. It means they agree at the tested markers to a degree that, statistically, is extremely unlikely to occur by chance for an unrelated person, and the laboratory reports that statistical weight rather than claiming absolute certainty.
Mixtures complicate the picture. A sample from a crime scene may contain DNA from several people, and separating those contributions requires careful analysis and, sometimes, probabilistic software that calculates likelihoods for different combinations. Courts expect experts to explain those likelihoods honestly, including the possibility that a mixture is too complex to interpret. A DNA result that is presented as simple and absolute, when the sample was mixed or degraded, should be treated with scepticism, because the science itself is more cautious than the headline.
Criminal Databases: The First Comparison
The first comparison for a new crime-scene profile is usually a criminal DNA database, which in many countries holds profiles from convicted offenders, arrestees where the law allows, and unidentified crime-scene samples. If the crime-scene profile matches a database profile, investigators gain a named lead, which they then verify with a fresh sample taken lawfully from the person. That verification step matters, because a database match is an investigative lead, not a conviction, and laboratory or record errors, though rare, must be ruled out.
Databases have solved many cold cases where the offender was later arrested for another crime and their DNA was entered into the system. A burglary arrest in one decade can therefore solve a murder from an earlier decade, if the same profile appears in both. That linkage is one reason police retain crime-scene profiles from unsolved cases: a profile that matches no one today may match someone tomorrow. Retention rules vary by country, and they are debated, because holding DNA from unsolved cases and from people never convicted raises privacy questions that legislatures continue to address.
Databases also have limits. They only contain people whose DNA was lawfully collected and entered, and many offenders have never been in the system. A cold case with no database match is not a dead end, but it requires a different method, and that is where forensic genealogy enters. The absence of a match should never be reported as proof that no DNA exists, only that no database profile matches the crime-scene profile at the tested markers.
Forensic Genealogy: Finding Family, Then a Suspect
Forensic genetic genealogy became widely known after the 2018 arrest of Joseph DeAngelo, identified through this method in the Golden State Killer case in California, a series of murders and assaults from the 1970s and 1980s. The method does not search a criminal database. Instead, investigators generate a broader genetic profile from crime-scene DNA, using many more markers than a standard forensic profile, and compare it with profiles in public genealogy databases where users have uploaded DNA results to find relatives. If a distant relative is found, genealogists build family trees forward and backward, looking for a person who fits the crime in age, sex, location, and other evidence.
That family-tree work is slow, detailed, and dependent on public records such as birth, marriage, death, census, and newspaper records. Genealogists may narrow hundreds of possible relatives to a small set of candidates, and investigators then look for independent evidence linking one candidate to the crime, such as a residence near the scene, a vehicle, or a prior record. The final step is always a direct DNA comparison: a lawfully obtained sample from the suspect is compared with the crime-scene profile using standard forensic markers. A genealogy lead alone does not charge or convict anyone.
The method has solved cases that databases could not, but it also raises privacy concerns. A person who uploads DNA to find ancestors may unknowingly make distant relatives identifiable to police, and those relatives never consented to that use. Some genealogy companies now allow users to opt in or out of law-enforcement matching, and police policies vary by country and agency. Courts and legislatures are still developing rules for when genealogy searching is appropriate, how far it may go, and what oversight is required, and readers should expect those rules to keep changing.
Retesting Old Evidence With New Technology
Not every cold-case breakthrough involves genealogy. Many come from retesting evidence with methods that did not exist, or were not sensitive enough, when the crime occurred. Touch DNA, which is DNA left by skin contact, can now sometimes be recovered from items that were previously thought to hold nothing, such as a steering wheel, a ligature, or a piece of clothing handled briefly. Improved extraction can recover DNA from degraded samples, and new laboratory techniques can separate a minor contributor from a mixture that once looked unreadable.
Retesting also benefits from better understanding of contamination. Early DNA work sometimes suffered from handling that mixed samples, and modern laboratories use stricter controls, protective clothing, and blank tests to detect contamination. When an old result is re-examined, scientists ask whether the original handling could explain it, and whether a new result is consistent with the crime or with later contact. A DNA profile on an item does not by itself prove when it was deposited or how, and that timing question is often central in court.
Families sometimes wonder why retesting takes so long. Laboratories face backlogs, cold cases compete with current cases for resources, and a single retest may involve several stages, each requiring review. Funding, staffing, and the condition of the evidence all affect timing. A case may also be retested more than once as methods improve, and each round must be documented so a court can follow the history of the sample. Patience is frustrating for families, but rushed testing that consumes a sample or produces an unclear result can be worse than waiting for the right method.
Building a Case Beyond the DNA Match
A DNA match, even a strong one, is only part of a prosecution. Investigators must still place the suspect at the scene at the relevant time, explain how the DNA could have arrived there, and address innocent explanations. DNA can be transferred indirectly, for example through contact with another person or object, and a profile on a shared item does not automatically prove presence at a crime. Prosecutors therefore combine DNA with witness accounts, timelines, vehicle and phone records, and other forensic evidence, and they test whether those pieces agree.
Defence examination is an essential part of this process. Defence experts may question the laboratory method, the statistical calculation, the handling history, or the interpretation of a mixture. They may also seek independent testing of remaining sample, where available. A case that survives that scrutiny is stronger than one built on a single laboratory report, and the adversarial process, though painful for families, helps prevent wrongful convictions based on misunderstood DNA evidence. Several wrongful convictions in history involved misinterpreted forensic evidence, which is why modern courts demand transparency about methods and limits.
For families, a DNA lead brings hope but not immediate closure. An arrest may be followed by months or years of legal proceedings, and a trial may revisit painful details. Some cases end in a guilty plea, some in a trial, and some in a decision not to charge because the evidence, even with DNA, does not meet the legal standard. Investigators who communicate honestly about those possibilities help families prepare, even when the news is disappointing. A DNA match is a major step, but it is not the same as a conviction, and it is not the end of the process.
Privacy, Consent, and the Rules Still Being Written
DNA is different from other evidence because it reveals family relationships and health-related information, even when investigators only use identity markers. A crime-scene profile can identify relatives who never committed a crime, and a genealogy search can expose family secrets, adoptions, or non-paternity events that families never intended to share. Those consequences are not hypothetical. Genealogists working on cold cases regularly encounter sensitive family information, and policies about what to disclose, and to whom, remain inconsistent.
Consent is the central debate. Supporters of genealogy searching argue that users who upload DNA to a public database, with terms that allow law-enforcement matching, have consented, and that solving violent crimes justifies the search. Critics argue that consent by one distant relative should not expose an entire family, and that police should need judicial authorisation for genealogy searches, similar to a warrant. Some jurisdictions now require a warrant or a court order, limit genealogy searching to violent crimes, or require that standard database searches be exhausted first. Readers should check the rules in their own country rather than assuming one country’s practice applies everywhere.
Data security is another concern. DNA databases, whether criminal or commercial, must protect profiles from breaches, misuse, and function creep, which means use beyond the original purpose. Laws governing retention, deletion, and access vary widely, and a profile entered for one purpose may be retained longer than the person expected. Public debate about these issues is healthy, because DNA technology will only become more powerful, and rules written now will affect cases for decades. Solving cold cases and protecting privacy are both legitimate goals, and the challenge is to pursue both without sacrificing either.
My Take:
My view is that DNA has changed cold cases from a waiting game into a genuine second chance, but only when it is used with humility. The most impressive part of a solved cold case is rarely the laboratory result alone. It is the combination of an old officer who preserved evidence, a laboratory that waited for the right method, a genealogist who built a careful family tree, and investigators who verified a lead with independent evidence. When any part of that chain is rushed or exaggerated, the result can mislead. I think the public should celebrate solved cases without turning DNA into a magic answer, because magic answers do not survive in court, and they do not help the next family whose sample is too degraded to test.
I also think we need to be honest about who benefits from DNA breakthroughs and who bears the cost. Victims’ families gain answers, and that matters enormously. At the same time, innocent relatives can be drawn into an investigation through genealogy, and their genetic information becomes part of a police inquiry they never chose. That trade-off is not resolved by saying crime-solving always wins, nor by saying privacy always wins. It requires clear rules: limit genealogy searching to serious violent crimes, require authorisation, exhaust standard databases first, and obtain a direct forensic match before any arrest. Those safeguards do not weaken good cases. They strengthen them, because a case built within clear rules is harder to challenge and more trustworthy to a jury.
Another point I feel strongly about is the risk of overclaiming what a DNA profile proves. A profile on an object proves contact or transfer, not necessarily guilt, timing, or intent. I have seen news reports that treat a match as a confession, and that shorthand does a disservice to everyone, including victims, because it creates expectations that a trial may not meet. If the defence shows an innocent explanation for the DNA, or if the mixture is too complex to interpret, the case may fail even though a match was reported. Better reporting would include the statistical weight, the sample type, and whether independent evidence supports the match, so readers can judge the strength of the case rather than just its headline.
I also want to acknowledge the families for whom DNA has not yet delivered an answer. For every celebrated solved case, there are many cold cases where the sample is too small, too degraded, or too mixed, or where no relative is in a genealogy database, or where the suspect died before identification. Those families watch other cases solved and wonder why theirs remains open, and that contrast can be painful. Investigators should explain, in plain language, what was tested, what was found, and what would need to change for a new attempt to be worthwhile, rather than leaving families with silence. Silence feels like abandonment, even when the real reason is a laboratory limit.
Finally, I believe the future of cold-case DNA work depends on trust, and trust depends on transparency about both successes and failures. Laboratories should publish their methods, courts should require clear explanations of statistics and mixtures, and genealogy companies should make law-enforcement matching an explicit, informed choice. If the public loses trust in DNA evidence because it was oversold or misused, then future victims will lose a powerful tool. The goal should be a system where a DNA match is exciting because it is careful, not because it is dramatic, and where every solved case strengthens confidence in the next one. That is slower than a headline, but it is the only version of progress that lasts.
One further issue deserves emphasis: the unequal distribution of DNA breakthroughs. Cases involving victims from well-resourced areas, with preserved evidence and active cold-case units, are more likely to benefit from retesting and genealogy than cases involving marginalised victims, poorly stored evidence, or underfunded departments. That inequality is not a criticism of any single laboratory, but it is a systemic problem, because justice should not depend on where a crime happened or who the victim was. Funding for evidence preservation, laboratory capacity, and cold-case review should be allocated with that fairness in mind, and departments should publish, in general terms, how they prioritise cold cases, so families can understand why one case moves faster than another. Transparency about prioritisation does not guarantee an answer, but it reduces the feeling that some victims matter less.
I also want to highlight the role of victims’ families as partners in cold-case DNA work, not merely as recipients of news. Families often hold photographs, letters, and memories that help genealogists and investigators, and they can provide reference DNA samples that help exclude relatives or identify remains. That cooperation requires trust, clear explanation of what a sample will be used for, how long it will be retained, and whether it could reveal family information beyond the case. When those terms are explained honestly and honoured in practice, families are more willing to help, and the investigation benefits. When they are vague or broken, trust collapses and future cooperation suffers. Consent, in cold-case work, is therefore not a bureaucratic step. It is the foundation of the partnership that makes many solutions possible.
A final reflection concerns the scientists themselves. Forensic work on a decades-old murder is emotionally demanding, technically exacting, and often invisible to the public, who see only the arrest headline. A laboratory scientist who spends months on a degraded sample, a genealogist who builds a family tree spanning centuries, and a detective who re-reads a file for the tenth time all contribute to a result that may never bear their names. Recognising that collective, patient labour matters, because it encourages the next generation to enter forensic science and cold-case work, and because it reminds us that justice in these cases is rarely the work of a single genius. It is the work of a system that preserves evidence, funds laboratories, respects privacy, and refuses to give up on families who have waited the longest. That system only works when each part is funded and respected, from the officer who bags evidence correctly on a rainy night to the analyst who checks a result twice before reporting it. Neglect any part, and the chain breaks, however powerful the science.
Frequently Asked Questions
How does DNA solve a cold case years later? Modern methods can read smaller, degraded, or mixed samples that older methods could not, and a profile that matched no criminal database entry may now match through forensic genealogy or through a later arrest. A laboratory first creates a profile from preserved evidence, compares it lawfully, and verifies any lead with a fresh sample.
What is forensic genetic genealogy in a cold case? It compares a broad genetic profile from crime-scene DNA with genealogy database profiles to find distant relatives, then builds family trees to narrow candidates who fit the crime’s time, place, and other evidence. A genealogy lead alone does not prove guilt; a direct forensic DNA comparison is still required.
Can DNA from an old crime scene be too degraded to use? Yes, heat, moisture, time, and poor storage can degrade DNA, and some samples are too small or too mixed to interpret reliably. Laboratories assess the sample, preserve part of it where possible, and may wait for improved methods rather than risk consuming the last material on an inconclusive test.
Does a DNA match prove someone committed the crime? No, a match shows that a person’s DNA profile agrees with the crime-scene profile at the tested markers, with a reported statistical weight, but it does not by itself prove timing, transfer route, or intent. Prosecutors combine DNA with timelines, witnesses, and other evidence, and defence experts may test innocent explanations.
Is police use of genealogy databases legal? Rules vary by country, company, and agency, with some requiring user opt-in, judicial authorisation, or limits to violent crimes after standard database searches are exhausted. Users should check the terms of the genealogy service they use and the law in their jurisdiction.
Why do some cold cases remain unsolved despite DNA? Some samples cannot yield a usable profile, no relative may be in a genealogy database, the suspect may be deceased or unidentifiable from records, or the DNA evidence alone may not meet the charging standard. Investigators may also face laboratory backlogs and limited funding for retesting.
