DNA testing for crimes began in 1986, when British police asked geneticist Alec Jeffreys to help identify a man who had raped and killed two girls in the English Midlands. Jeffreys had developed the first DNA profiling technique two years earlier, in 1984, at the University of Leicester. That case, which led to the conviction of Colin Pitchfork, became the first criminal investigation solved through DNA evidence and launched a forensic revolution that now underpins criminal justice systems worldwide.
How DNA Profiling Was Invented
In 1984, Alec Jeffreys and his colleagues discovered that certain regions of human DNA vary enormously from person to person. These variable stretches, called restriction fragment length polymorphisms, created patterns unique enough to function like a molecular fingerprint. Jeffreys realized that comparing these patterns could identify individuals with near-certainty, and he developed a laboratory method to visualize the differences. The technique was originally intended for paternity testing and immigration disputes, not crime scenes. Its forensic potential became obvious almost immediately.
The Case That Proved It Worked
In 1986, police in Leicestershire, England, were investigating the murders of two teenage girls, killed in 1983 and 1986. They had a prime suspect who had confessed to one of the killings. When Jeffreys tested the suspect’s DNA against biological evidence from both crime scenes, the results were stunning: the suspect’s DNA matched neither victim’s evidence. He became the first person ever exonerated by DNA testing.
Police then organized what amounted to a genetic dragnet, asking thousands of local men to voluntarily provide blood samples for testing. The real killer, Colin Pitchfork, tried to evade the dragnet by persuading a friend to submit a sample in his place. When the friend mentioned the swap at a pub, someone reported it to police. Pitchfork’s DNA matched both crime scenes, and he was convicted in 1988. Within a year of that case, DNA profiling was being adopted by criminal investigators around the world.
Early Limitations of DNA Testing
The original technique required a substantial amount of biological material to work. Crime scene samples needed to be roughly the size of a dime or a quarter, like a visible bloodstain, to yield enough DNA for profiling. The process was also slow and technically demanding, requiring large quantities of high-quality DNA that hadn’t been degraded by heat, moisture, or time. Many crime scene samples simply weren’t usable.
These limitations meant that DNA evidence in the late 1980s was powerful but narrow. It could confirm or rule out a suspect when good biological evidence existed, but it couldn’t do much with trace amounts of material, partial samples, or evidence that had been exposed to the elements.
The Shift to Modern DNA Methods
In the 1990s, forensic labs began adopting a technology called PCR, which can copy tiny fragments of DNA millions of times over. This was transformative. Instead of needing a visible bloodstain, analysts could now work with far smaller and more degraded samples. The method also switched from analyzing large variable regions of DNA to focusing on shorter repeating sequences, which survive better in damaged or old evidence.
The practical difference is enormous. Today, investigators can retrieve a DNA profile from skin cells left behind when someone merely touches a surface. A doorknob, a steering wheel, the trigger of a gun: all of these can yield usable DNA. That sensitivity has opened up categories of crime, like burglaries and car thefts, that were once far too minor to warrant DNA analysis.
Building a National DNA Database
The FBI launched its Combined DNA Index System, known as CODIS, as a pilot program in 1990, connecting 14 state and local laboratories. Four years later, the DNA Identification Act of 1994 gave the FBI formal authority to operate a National DNA Index System for law enforcement. The idea was simple but powerful: store DNA profiles from convicted offenders in a searchable database so that crime scene evidence could be checked against known individuals automatically.
As of November 2025, that database holds over 19.2 million offender profiles, 6.1 million arrestee profiles, and nearly 1.5 million forensic profiles from unsolved cases. When investigators recover DNA from a crime scene, they can run it against the entire national index. Cold cases that sat unsolved for decades have been cracked this way, sometimes matching evidence from the 1980s or 1990s to offenders who entered the database years later for unrelated crimes.
DNA’s Role in Freeing the Innocent
DNA testing didn’t just help catch criminals. It also exposed how often the justice system convicted the wrong person. Kirk Bloodsworth was sentenced to death in Maryland for the 1984 sexual assault and murder of a nine-year-old girl, convicted largely on eyewitness testimony and an anonymous tip. His attorney, working with the Innocence Project, eventually persuaded officials to compare Bloodsworth’s DNA against biological evidence preserved from the crime scene. The results cleared him completely, and he was freed in June 1993, becoming the first death-row prisoner exonerated by post-conviction DNA testing.
Bloodsworth’s case helped establish a model that has since been repeated hundreds of times. Organizations like the Innocence Project have used DNA analysis to overturn wrongful convictions across the country, revealing patterns of flawed eyewitness identification, false confessions, and unreliable forensic methods that preceded the DNA era. The technology that began as a tool for catching killers turned out to be equally important as a safeguard against injustice.