How Cold Cases Actually Get Solved
Eight ways back into an old file — what each requires, what it can prove, and where it tends to fail.
Cold-case accounts often lean on the phrase new technology and move on. That hides most of the work. A CODIS match, investigative genetic genealogy, evidence retesting and records digitisation are different procedures with different costs and failure points. A witness who talks after thirty years is something else again.
An old case moves when something specific changes. A sample has survived in testable condition. A database has grown. A relationship has ended. Someone has finally indexed the box that nobody could search.
The mechanism matters because it tells us what investigators can try again — and what time has already taken away.
Eight Ways Back Into a Cold Case
There is no national scoreboard. A RAND study for the National Institute of Justice found that agencies often fail to track even basic outcomes such as filings, convictions, sentences and time spent per clearance. The available ranking has to be pieced together from grant results, agency surveys and expert reviews.
The order below reflects that incomplete record. It is a useful comparison, not a league table.
| Mechanism | What it needs | What it delivers |
|---|---|---|
| Preserved-evidence retesting | Surviving biological evidence, intact chain of custody | The largest formal engine of grant-backed cold-case work |
| Genetic genealogy | A usable sample, SNP testing, prosecutor concurrence | Identification where no database hit exists — fast-growing, still a branch |
| Witness relationship changes | Living witnesses whose incentives have shifted | Low cost per case, disproportionately high conviction rate |
| Informants and cooperators | An insider with something to trade | Decades-old charges revived — needs heavy corroboration |
| Deathbed and posthumous confessions | A confession plus verifiable non-public detail | Frequently solves; frequently cannot prosecute |
| Archival digitization | Paper files, scanning capacity, indexing | Rarely the proof; often the reason a review is possible at all |
| Fingerprint and ballistic advances | Latent prints or cartridge cases, expanded databases | Investigative leads and corroboration, not certainty |
| Renewed public visibility | An anniversary, a documentary, a campaign | The least common route — an accelerant, not a solution |
Preserved-Evidence Retesting
Largest formal engineVerify the evidence still exists, rebuild the chain-of-custody record, identify which items are worth modern testing, retest with methods that did not exist at the time, and upload any resulting profile to CODIS.
A match gives investigators a name to examine. They still have to account for degraded evidence, dead suspects and whatever gaps remain in the prosecutor's case.
Investigative Genetic Genealogy
Fastest-growing branchUsed only after conventional DNA routes stall: secure a crime-scene sample, run SNP testing suitable for genealogy, upload to a law-enforcement-permitted platform, identify distant relatives, build trees, narrow by age, sex, geography and opportunity, then confirm with a direct reference sample.
The result is a candidate who must be confirmed by direct comparison. Department of Justice policy largely reserves the method for violent crime and unidentified remains, and the workflow now includes prosecutor concurrence.
Witness Relationship Changes
Most potent per caseReturn to the people who once minimised, denied or compartmentalised what they knew — and to those whose incentives have since changed through divorce, estrangement, immunity, age, remorse, or the simple fact that the suspect no longer has power over them.
RAND found these cases relatively cheap to pursue compared with a full re-review. When the new account can be substantiated, it may support an exceptional clearance and often a conviction. The category is small, but productive.
Informants and Cooperators
Efficient, needs corroborationSomeone with inside knowledge — an accomplice, an associate, a jail source — trades information for leniency. Investigators weigh the motive, corroborate non-public details against physical evidence, and use what survives that test to obtain warrants, pleas or admissions from others.
A cooperator's word has little value by itself. Investigators look for unpublished details and then check those details against the physical record.
Deathbed and Posthumous Confessions
Solves, rarely prosecutesA suspect, relative or confidant divulges incriminating information near death or after the suspect has died. The confession is treated as a lead: does the speaker know facts that were never public, and can remains or physical evidence corroborate them?
A confession may settle what happened without making a prosecution possible. By the time it arrives, the person who could have been tried is often dead.
Archival Digitization
Enabling infrastructureGather paper files scattered across departments, standardise and scan them, index or OCR the text, and turn the result into something searchable — which surfaces forgotten names, missed testing opportunities, cross-case links and jurisdictional overlaps.
Digitisation rarely supplies the proof. It makes the review possible by opening old files to linkage searches and text analysis. NIJ therefore treats it as investigative infrastructure, not clerical housekeeping.
Fingerprint and Ballistic Advances
Lead generator, not certaintyRecover latent prints or palmprints from old evidence and resubmit them into automated systems that have grown enormously since the file went cold; on the ballistics side, compare cartridge-case markings through NIBIN to link scenes and recovered weapons.
An automated search returns a candidate for an examiner to compare and investigators to corroborate. Caution is warranted: NIJ has linked false or misleading forensic evidence to wrongful convictions, and federal reviews have criticised overclaiming in pattern-comparison fields.
Renewed Public Visibility
Least common routeBring the case back into public view through an anniversary, documentary, book, podcast or reward campaign, so that people who once ignored, forgot or feared it now come forward — and so that agencies reallocate attention to the file.
RAND recognises family- and media-driven reopening as a real category, but the least common one. It also tends to be costly and unsuccessful unless the attention prompts a witness, a test or a funded review.
The Numbers, Where They Exist
NIJ's Solving Cold Cases With DNA programme provides the largest published set of figures:
| Stage | Cases |
|---|---|
| Unresolved cases reviewed | 141,371 |
| Biological evidence still present | 34,289 |
| DNA actually tested | 14,371 |
| CODIS uploads | 5,580 |
| CODIS hits | 2,061 |
| Trials, arrests or closure | 2,085 |
The loss at each stage is substantial. Only about one reviewed case in four still contained biological evidence. Fewer than half of those samples were tested. By the end of the process, 2,085 of 141,371 reviewed cases had produced a trial, an arrest or a closure. Most files dropped out long before a laboratory result was possible.
DNA was not the decisive tool everywhere. In Charlotte-Mecklenburg, investigators and prosecutors estimated that it would probably not be central to more than half of homicide or sexual-assault prosecutions. RAND's survey found a stronger association between clearance rates and access to investigative databases. That finding receives less attention, perhaps because database access makes a poor television reveal.
What the Familiar Case Lists Leave Out
New equipment does not make a file solvable by itself. RAND found that the reason for reopening, the quality of the original investigation, the possibility of a new theory, the availability of witnesses and a prosecutor's willingness to proceed all matter alongside technology.
A DNA match does not guarantee charges. Between a third and a half of successful investigations at RAND's study sites produced no arrest. Even among sexual-assault cases with a suspect match, roughly a third were not filed because the suspect was dead or unavailable, or because cooperation and credibility problems remained.
Publicity can bring tips, money and administrative attention, but RAND found media-driven reopening to be the least common route. The case still needs a witness, a testable sample or another piece of admissible evidence.
Old forensic conclusions also deserve re-examination. NIJ has connected false or misleading forensic evidence with wrongful convictions, while federal science reviews have challenged historical claims made for bullet and cartridge-case comparison.
Nor is digitisation merely filing. Searchable records can expose missed testing opportunities and links between cases. NIJ guidance tells agencies to plan scanning and indexing around the questions a review is meant to answer.
A small chronology error shows how easily the familiar story gets compressed. One prominent page says Michelle McNamara's work became an HBO series and that police arrested Joseph DeAngelo two months after its release. DeAngelo was arrested in April 2018; I'll Be Gone in the Dark premiered in 2020. The arrest, book, documentary and DNA investigation had been folded into a cleaner story than the dates allow.
Quick Answers
What is the difference between CODIS and genetic genealogy?
CODIS is a law-enforcement DNA database of offender and casework profiles; it works when the perpetrator — or a direct profile — is already in the system. Genetic genealogy is used when that fails: SNP-style DNA is searched against permitted genealogy databases for distant relatives, family trees are built down to a candidate, and the candidate is then confirmed by direct comparison.
Can a cold case be solved without DNA?
Yes, routinely. Latent-print automation, palmprint comparison, witness cooperation, informants and confessions all still close cases — the Carroll Bonnet and Rodney Wyman murders were both reopened through fingerprint searches, not DNA.
Do podcasts and documentaries really solve cases?
Sometimes. They can generate tips, prompt an agency review or reach a witness who had stopped paying attention to the case. A prosecution still depends on evidence that will survive in court.
Are deathbed confessions enough on their own?
No. Investigators still need corroboration — knowledge of non-public facts, recovered remains, matching physical evidence. The Carter case is the model: the confession mattered because it led to a burial site that could be verified.
What matters more, a new test or a new witness?
Usually both. Witness-driven openings are unusually efficient when they occur; forensic retesting is the largest formal driver in most agencies' portfolios. The most persuasive resolutions are hybrids, where a scientific lead and human testimony reinforce each other.
Cold cases reopen for specific reasons: a sample survives, a database changes, an old file becomes searchable, or a witness is ready to give a different account.
None of those developments closes the case on its own. They give investigators a reason to take the folder down again and see what can still be proved.