How Cold Cases Get Solved: The Forensic Techniques That Crack Decades-Old Mysteries

Joseph James DeAngelo was 72 years old when they came for him. He was mowing his lawn in Citrus Heights, California — a quiet Sacramento suburb — on April 24, 2018. A regular old man doing regular old-man things. Except Joseph James DeAngelo was the Golden State Killer. Thirteen murders. Over fifty rapes. Terrorized entire communities across California for more than a decade, from 1974 to 1986. He was a former police officer. He had a family. He'd been hiding in plain sight for forty years.

Forty years. Forty years this guy walked free, living a mundane suburban life while the families of his victims buried their dead and carried nightmares that never faded. And it ended because a detective named Paul Holes uploaded crime scene DNA to a free, public genealogy database called GEDmatch — the kind of site where people go to find out they're 12% Scandinavian — and matched it to a distant relative's family tree.

Not DeAngelo's DNA. A relative's. A third cousin, maybe a fourth. Someone who'd swabbed their cheek and uploaded their genetic profile because they were curious about their ancestry. That was enough. Investigators built out the family tree, branch by branch, generation by generation, until they narrowed it down to one man in one house in Citrus Heights. They collected a DNA sample from something DeAngelo discarded — the exact item has never been publicly confirmed, though some reports say it was a tissue from his trash — and it matched the crime scene evidence.

Forty years. And a cotton swab from a genealogy website brought him down.

What makes a case go cold

A cold case isn't an abandoned case. That distinction matters. Detectives don't close a murder file and toss it in the shredder because they couldn't solve it in six months. The case gets shelved. The physical evidence — blood samples, hair fibers, clothing, fingerprints, photographs, whatever was collected at the scene — goes into a box. That box goes into a warehouse. And it sits there.

Sometimes for years. Sometimes for decades.

Cases go cold for a lot of reasons. Insufficient physical evidence at the time. No eyewitnesses, or eyewitnesses whose accounts contradicted each other. A suspect who was pursued aggressively but turned out to be innocent, wasting months of investigative momentum. Overwhelmed police departments in cities where a new homicide lands on the desk before the last one is solved. Witnesses who moved away. Witnesses who died. Detectives who retired. Detectives who died.

But here's the thing that makes cold cases different from every other kind of failure: the evidence doesn't expire. DNA doesn't have a statute of limitations. A fingerprint from 1982 is still a fingerprint. A blood-soaked shirt sitting in an evidence locker for thirty years still carries the genetic signature of whoever bled on it. The case file is still there, every interview transcript, every crime scene photo, every dead-end lead — waiting for someone with better tools or a fresh pair of eyes.

The case isn't dead. It's sleeping.

Forensic genealogy: the game-changer

If there is a single technology that has revolutionized cold case investigation in the 21st century, it's forensic genealogy. Nothing else comes close. Since the Golden State Killer arrest in 2018, this technique has solved over 200 cold cases, many of them decades old. It has identified both killers and victims — sometimes people who were buried as John or Jane Doe for generations finally got their names back.

Here's how it works.

Investigators extract DNA from crime scene evidence — blood, semen, saliva, skin cells, whatever survived. They generate a DNA profile and upload it to a public genealogy database like GEDmatch or FamilyTreeDNA. These databases contain genetic profiles voluntarily submitted by millions of people interested in their ancestry. The crime scene DNA doesn't need to match the killer directly. It just needs to match a relative.

And here's where it gets remarkable. The match doesn't even need to be close. A third cousin. A fourth cousin. Someone who shares a great-great-great-grandparent with the killer. That's enough to start. From that single match, investigators — often working with genetic genealogists from companies like Parabon NanoLabs — begin building out the family tree. They work forward and backward through generations, cross-referencing census records, birth certificates, marriage records, obituaries, and public records. They narrow down branches of the tree based on age, sex, geography, and any other factors from the case.

Eventually, they land on a suspect. Then they confirm it the old-fashioned way — collect a DNA sample (from discarded trash, a coffee cup left at a restaurant, a door handle) and match it directly to the crime scene evidence.

The cases this technique has cracked are staggering.

April Tinsley. Eight years old. Abducted and murdered in Fort Wayne, Indiana, in 1988. Her killer left taunting notes to police over the years but was never identified. In 2018 — thirty years later — forensic genealogy matched crime scene DNA to a man named John Miller. He confessed.

The Bear Brook murders. Four bodies — a woman and three girls — found in barrels in Bear Brook State Park, New Hampshire, in 1985 and 2000. Nobody knew who they were. Nobody knew who killed them. In 2019, forensic genealogy identified both the victims and the killer. The woman was Marlyse Honeychurch. Two of the girls were her daughters. The killer was Terry Rasmussen, a serial killer who had died in prison in 2010 under a different name. Genealogy didn't just solve the murder — it gave four nameless people their identities back.

Two hundred cases and counting. Murders that sat unsolved for 10, 20, 30, even 50 years. All cracked because someone's distant cousin decided to take a genealogy test.

DNA retesting with modern technology

When forensic DNA profiling entered criminal investigation in the mid-1980s, it was revolutionary but crude by today's standards. You needed a sample the size of a quarter. It had to be relatively fresh and uncontaminated. Many crime scenes simply didn't yield usable DNA with the technology available at the time. A partial fingerprint? Useless. A hair without the root? Useless. Skin cells on a surface? Forget it — nobody even knew that was possible.

Now look at what we can do.

Touch DNA is exactly what it sounds like — DNA extracted from skin cells left behind when someone touches a surface. A doorknob. A zipper. A steering wheel. The amount of genetic material is vanishingly small, but modern PCR (polymerase chain reaction) amplification can take those few cells and produce a full DNA profile. Evidence that was collected in 1990 and stored in an evidence locker — evidence that was completely useless at the time — can now be retested and yield a match.

Mitochondrial DNA (mtDNA) analysis has opened another door. Nuclear DNA — the kind used in standard profiling — degrades over time. Mitochondrial DNA, which is passed down through the maternal line, is far more resilient. It survives in skeletal remains, in hair shafts without roots, in samples that are decades or even centuries old. It's less specific than nuclear DNA (it identifies a maternal lineage rather than a unique individual), but in cold cases where evidence is degraded, it can be the difference between nothing and a lead.

Then there's the M-Vac system — and this one is genuinely cool. It's a wet-vacuum collection device that can pull DNA off surfaces that traditional swabbing can't handle. Rough surfaces. Porous surfaces. Rocks. Bricks. Denim fabric. Items where the DNA is embedded in the texture of the material and a cotton swab just slides over it. The M-Vac sprays a sterile solution onto the surface and immediately vacuums it back up, collecting biological material along with it. Cases that had zero usable DNA when processed in the 1990s have been retested with the M-Vac and suddenly produced full profiles.

A single hair. A partial fingerprint. A stain on fabric that nobody could test thirty years ago. Today, any one of these can become a conviction.

Digital forensics on old devices

This is one that people don't think about as much, but it's increasingly relevant. As cases from the late 1990s and 2000s enter cold case territory, the evidence lockers contain something that didn't exist in earlier decades: digital devices.

Old computers. Flip phones. Digital cameras. PDAs. USB drives. CDs burned in 2003 and tossed into an evidence bag. At the time they were collected, the technology to fully examine them may not have existed — or the investigators may not have had the tools, training, or budget to do it properly.

Modern digital forensics has changed that equation entirely. Deleted files can be recovered from hard drives, even if someone thought they erased them. When you delete a file, most operating systems don't actually destroy the data — they mark the space as available to be overwritten. Until something else writes over that exact spot on the disk, the original data is still there. Forensic software can extract it.

Old security camera footage from analog systems has been digitized and enhanced using modern image processing. Grainy, low-resolution video that was unwatchable in 2002 can be sharpened enough to identify a face or read a license plate. Cell tower records — which phone companies are required to retain for varying periods — can be re-analyzed to place a suspect at a location with far more precision than was possible when the records were first pulled.

A murder case from 2004 might have a suspect's Nokia brick phone sitting in evidence. At the time, extracting data from it was difficult and expensive. Now? A forensic analyst can pull text messages, call logs, location data, and even deleted content from that phone in an afternoon. The evidence was always there. The tools just had to catch up.

Re-interviewing witnesses: people talk eventually

This one isn't about technology at all. It's about human nature. And it might be the most underrated cold case technique there is.

People change. Relationships end. Loyalties shift. Fear fades. Guilt grows.

The girlfriend who provided a false alibi in 1997? She's an ex-wife now. She has no love left for the man she covered for, and she's been carrying the weight of that lie for twenty-five years. The friend who saw something and stayed quiet because he was terrified? He's older now. Maybe he's sick. Maybe he's dying. Maybe he just can't carry it anymore and he wants to tell the truth before he's gone.

Cold case detectives know this. It's one of the first things they do when they reopen a case — go back to every witness, every associate, every person who was even peripherally connected to the victim or the suspect, and ask again. Twenty years changes people. The story that was airtight in 1998 has holes in it now, because the person telling it no longer has a reason to lie.

Sometimes it's not even about deception. Memory is strange. A witness who genuinely couldn't remember a detail in the immediate aftermath of a traumatic event sometimes recalls it years later, triggered by something unexpected. A smell. A song. A location they drive past for the thousandth time and suddenly remember what they saw there.

And sometimes people just want to talk. They've been holding something for decades and they're looking for permission to let it go. A patient detective who shows up at their door and says "I'm still working on this — it still matters" can be enough.

Crowdsourcing and public attention

The rise of true crime media — podcasts, documentaries, online communities — has done something that no one really predicted. It's turned millions of ordinary people into amateur investigators. And sometimes, controversially, it works.

The Kristin Smart case is the clearest example. Kristin Smart was a 19-year-old Cal Poly student who disappeared in 1996. The case went cold for decades, despite a prime suspect — Paul Flores, the last person seen with her — being known to police almost immediately. In 2019, journalist Chris Lambert launched a podcast called Your Own Backyard, which meticulously re-examined the case and brought massive public attention to it. Tips poured in. New witnesses came forward. In 2021, Paul Flores was arrested and charged with murder. In 2022, he was convicted. Lambert's podcast didn't just raise awareness — it generated new evidence that directly contributed to the prosecution.

Online communities like Reddit's r/UnresolvedMysteries, with over 3 million members, have become hubs for crowdsourced investigation. Citizen detectives pore over case files, spot inconsistencies, generate leads, and occasionally identify victims or suspects that professional investigators missed. The Grateful Doe — an unidentified young man killed in a car accident following a Grateful Dead concert in 1995 — was identified in 2015 after internet sleuths matched a reconstruction to a missing persons report.

It's not without serious problems. The Reddit "investigation" of the Boston Marathon bombing in 2013 is the cautionary tale — internet users wrongly identified multiple innocent people as suspects, harassing their families. One person they named, Sunil Tripathi, was already dead; his family endured a nightmare of false accusations while grieving. Citizen detective work can be powerful, but without the discipline, training, and accountability of professional law enforcement, it can also destroy innocent lives.

When it works, though — when public attention shakes loose a tip that's been buried for twenty years, when a podcast reaches the one person who knows something and gives them the courage to call — it's extraordinary. The truth was always out there. Sometimes it just needed a bigger audience to find it.

The detectives who never gave up

Behind every cold case that gets solved, there's a human being who refused to let go.

Cold case units exist in major police departments across the country, but they're almost always understaffed and under-resourced. The detectives who work them are often volunteers — experienced investigators who could be doing other work but choose to spend their time with cases that everyone else has given up on. Some of them inherit a case and work it for five years. Some work it for twenty.

Paul Holes — the detective who caught the Golden State Killer — spent 24 years on that case. He was weeks away from retirement when the genealogy match came through. Twenty-four years. He'd studied the crime scenes. He'd memorized the victimology. He'd read every report, listened to every recording, visited every location. And when the new technology finally existed to crack it, he was still there. Still working it.

There's a particular kind of obsession that cold case detectives carry. These aren't just file numbers to them. They know the victims' names, their families, their stories. They've sat across from mothers who lost daughters and promised to keep looking. They've held evidence in their hands that they know contains the answer, if only the science could catch up. And they wait. They push. They refuse to close the file.

When a case finally breaks — when the DNA matches, when the witness calls, when the genealogy tree narrows to one name — the detective who worked it for years or decades doesn't just feel professional satisfaction. They feel relief. They feel it for the victim. They feel it for the family. They feel it for themselves, because the case lived in their head every single day and now, finally, there's an answer.

Not every cold case gets solved. Most don't. But the ones that do — they get solved because someone cared enough to keep the light on.

Investigating from the detective's chair

DetectiveOS puts you in that chair. Every case is built like a real cold case investigation — evidence that's been sitting untouched, witnesses whose stories don't add up, forensic tools that can reveal what was missed the first time around. You're not watching a documentary. You're the one connecting the dots.

The evidence doesn't expire. The case isn't closed. Someone just has to care enough to open the box again.