DNA evidence helps solve crimes by matching biological traces from a scene to a specific person, linking separate unsolved cases to a common offender, putting names to unidentified remains, and clearing people convicted of crimes they did not commit. The FBI’s Combined DNA Index System alone has produced more than 781,000 hits across over 758,000 investigations.1Federal Bureau of Investigation. CODIS-NDIS Statistics A single drop of blood, a few skin cells on a doorknob, or a decades-old sexual assault kit can now generate a genetic profile precise enough to identify one person out of millions.
How a Sample Becomes a Profile
About 99.9% of human DNA is identical between any two people. Forensic testing targets the remaining 0.1%, focusing on short tandem repeats (STRs): short DNA sequences that repeat a variable number of times at specific locations in the genome. Testing enough of these locations makes the odds of two unrelated people sharing the same profile vanishingly small. Identical twins are the only exception.2National Institute of Justice. What Is STR Analysis
Analysts extract DNA from the biological material, measure how much is present, and use polymerase chain reaction (PCR) to make millions of copies of the target regions. The resulting profile is a set of numbers representing repeat counts at each STR location. That numeric fingerprint is what gets compared against known suspects or uploaded to a database.
Matching Suspects Through CODIS
Crime scenes yield DNA from blood, saliva, semen, hair, and skin cells. Even brief contact leaves usable material. Forensic analysts call DNA recovered from handled objects like weapons, steering wheels, and window frames “touch DNA,” and modern methods work with remarkably small quantities.3National Institute of Justice. Crime Scene Integrity – Reducing the Risk of Contamination
When a crime scene profile matches a known suspect, it places that person at the scene with near-certainty. When there’s no suspect, investigators upload the profile to CODIS, the FBI’s national database of profiles from convicted offenders, arrestees in participating jurisdictions, and forensic samples from unsolved cases.4Federal Bureau of Investigation. CODIS and NDIS Fact Sheet
Two kinds of leads come out of the system. An offender hit occurs when crime scene DNA matches someone already in the database. A forensic hit occurs when two unsolved crime scene profiles match each other, revealing that separate cases share a perpetrator.5National Institute of Justice. What Every Investigator and Evidence Technician Should Know About DNA Evidence – Database Files As of November 2025, CODIS held more than 19.2 million offender profiles, 6.1 million arrestee profiles, and 1.4 million forensic profiles.1Federal Bureau of Investigation. CODIS-NDIS Statistics
Reopening Cold Cases
Advances in DNA technology have revived investigations that stalled years or decades ago. Samples once considered too degraded, too small, or too complex for older methods can now yield usable profiles. Mitochondrial DNA analysis works when nuclear DNA is too damaged because mitochondrial DNA is present in far greater quantities per cell. Y-STR analysis targets the male-specific Y chromosome and is useful when standard testing fails, such as with fingernail scrapings from a female victim who scratched a male assailant.6National Institute of Justice. Principles of Forensic DNA for Officers of the Court – Indications for Y-STR Analysis
When a newly generated profile enters CODIS and matches a convicted offender or another unsolved case, the resulting cold hit can break a case investigators had given up on. The most dramatic development in cold case work, though, has been forensic genetic genealogy.
Forensic Genetic Genealogy
Forensic genetic genealogy (FGG) uploads a crime scene profile not to law enforcement databases but to public genealogy databases populated by people who voluntarily submitted their DNA to research their ancestry. Genealogists identify distant relatives of the unknown contributor and build family trees outward. Traditional detective work — ages, locations, biographical details — then narrows the field to a single person.
The technique made headlines in 2018 when California investigators used GEDmatch to identify relatives of the Golden State Killer, a serial offender linked to at least 13 murders and 50 sexual assaults across the 1970s and 1980s. The arrest came more than 40 years after the crimes began. Three genealogy databases currently allow law enforcement uploads: GEDmatch, FamilyTreeDNA, and DNASolves.7National Center for Biotechnology Information. Bridging Disciplines to Form a New One – The Emergence of Forensic Genetic Genealogy
Under a Department of Justice interim policy, federal investigators and certain grant recipients may use FGG only for unsolved violent crimes or unidentified human remains, only after CODIS has failed to produce a match, and only after other reasonable leads have been pursued.8United States Department of Justice. Interim Policy – Forensic Genetic Genealogical DNA Analysis and Searching State and local agencies operate under fewer restrictions.
Identifying Victims and Unknown Remains
When visual recognition, dental records, and fingerprints are not options, DNA is often the only route to an identification. After mass disasters, severe fires, or prolonged decomposition, remains may be unrecognizable. Bones and teeth preserve genetic material comparatively well, and long bones like the femur and tibia tend to be the best sources because their hard exterior shields the DNA inside.9National Library of Medicine. Comparison of Three Methods of DNA Extraction From Human Bones With Different Degrees of Degradation
The National Missing and Unidentified Persons System (NamUs), funded by the National Institute of Justice, coordinates DNA work in these cases. Profiles from unidentified remains go into a dedicated CODIS index and are compared against profiles from missing persons and their relatives. Family samples submitted for this purpose are searched only against the unidentified remains index, not against criminal databases.10National Institute of Justice. DNA Analysis and CODIS Searching
First-degree relatives — parents, full siblings, and children — produce the strongest matches. NamUs recommends collecting DNA from at least two family members per missing person case to improve the odds of a successful search. Personal items belonging to the missing person, such as a toothbrush or hairbrush, can also supply a reference sample.10National Institute of Justice. DNA Analysis and CODIS Searching
Clearing the Wrongly Convicted
The same technology that identifies perpetrators has exposed serious failures inside the justice system. Through 2020, at least 375 people in the United States were exonerated by DNA evidence after being convicted of crimes they did not commit, having served an average of 14 years in prison.11Innocence Project. DNA Exonerations in the United States 1989-2020 The process usually involves retesting original crime scene evidence, or testing evidence that was never analyzed the first time, and comparing the result against the convicted person’s DNA. When the profiles do not match, the exclusion is definitive.
Eyewitness misidentification drove roughly 69% of these wrongful convictions. False confessions, unreliable forensic disciplines, and informant testimony contributed as well. In many cases, the DNA that freed the innocent person also produced a CODIS match to the actual perpetrator, who had sometimes continued committing crimes for years while someone else served the sentence.
Exonerations depend on evidence still existing. Federal law requires the government to preserve biological evidence, including sexual assault kits, blood, saliva, hair, and skin tissue, whenever a defendant is sentenced to prison for a federal offense.12Office of the Law Revision Counsel. 18 USC 3600A – Preservation of Biological Evidence Federal law also lets imprisoned individuals petition for DNA testing if they assert actual innocence, the evidence was not previously tested or better methods now exist, chain of custody has been maintained, and the results could raise a reasonable probability that the applicant did not commit the offense.13Office of the Law Revision Counsel. 18 USC 3600 – DNA Testing Most states have enacted their own preservation and post-conviction testing statutes.
Where DNA Evidence Falls Short
DNA is powerful, but it does not deliver the certainty popular fiction suggests. Secondary transfer is one of the harder problems: DNA can move from one person to an object or location through an intermediary, without the original person ever being there. A handshake can deposit enough genetic material for testing. Research shows DNA can pass through as many as six intermediary transfers, in as little as ten seconds. Labs can detect trace DNA on a surface, but they cannot determine how or when it arrived.
Contamination is a related risk. Because modern testing is extraordinarily sensitive, small lapses during collection — eating near a scene, failing to change gloves between items, packaging evidence improperly — can introduce DNA from someone unrelated to the crime. The National Institute of Justice stresses that DNA evidence is more sensitive to contamination than other physical evidence and requires unusually careful handling.3National Institute of Justice. Crime Scene Integrity – Reducing the Risk of Contamination
Processing delays cause their own problems. DNA backlogs remain a persistent issue nationally. Estimates of untested sexual assault kits alone run from tens of thousands to hundreds of thousands, and many jurisdictions fail to test the majority of submitted evidence within 30 days. Cases go cold while evidence sits in storage, and the longer a case waits, the harder it becomes to investigate once results arrive.
Courts also scrutinize DNA evidence before it reaches a jury. In federal courts, judges evaluate scientific testimony under the framework from Daubert v. Merrell Dow Pharmaceuticals (1993), which asks whether the underlying methods are testable, peer-reviewed, and widely accepted in the relevant scientific community. Defense attorneys challenge DNA evidence on grounds of contamination, broken chain of custody, or flawed lab procedures, and those challenges occasionally succeed when documentation is weak or protocols were not followed.
What’s Changing Next
Rapid DNA
Rapid DNA instruments generate a profile from a cheek swab during booking, producing results in under two hours instead of the weeks or months required by traditional lab analysis. The Rapid DNA Act of 2017 authorized the FBI to set standards for these instruments, and qualifying arrestee profiles can now be uploaded directly to CODIS from booking stations.14Federal Bureau of Investigation. National Rapid DNA Booking Operational Procedures Manual An arrested person’s DNA can be compared against unsolved sexual assault, homicide, kidnapping, and terrorism cases while they are still in custody. The technology is currently authorized only for booking samples; crime scene evidence still goes through a traditional laboratory.
Next-Generation Sequencing
Massively parallel sequencing, also called next-generation sequencing, expands what forensic labs can extract from difficult samples. It handles degraded DNA more effectively by targeting shorter fragments, and it separates multiple contributors in mixed samples far better than older methods. Some platforms can detect a minor contributor representing as little as 1% of a mixture, and newer markers called microhaplotypes can differentiate mixtures with as many as six donors.15National Center for Biotechnology Information. Applications of Massively Parallel Sequencing in Forensic Genetics For crime scenes where several people’s DNA is intermingled, common in sexual assault cases, this is a significant advance.
DNA Phenotyping
Instead of matching DNA to a known person, phenotyping predicts an unknown person’s likely appearance from their genetic material. Eye color, hair color, skin tone, and biogeographic ancestry can all be estimated from a crime scene sample.16National Academies of Sciences, Engineering, and Medicine. Forensic DNA Phenotyping Phenotyping does not identify a specific individual. It produces a probabilistic description that helps narrow a suspect pool when no other leads exist, functioning as a rough sketch drawn from genetic code rather than an eyewitness account.