What Is a GSK Killer? How Forensic Science Cracked the Case
Updated September 22, 2026
GSK stands for Golden State Killer, the name for a serial offender tied to at least 13 murders, at least 50 rapes, and at least 120 burglaries in California between 1974 and 1986. He stayed unidentified until 2018, when genetic genealogy pointed investigators to Joseph DeAngelo, a 72-year-old former police officer, and his own discarded DNA confirmed the match.
You’ve probably seen the GSK case referenced in a true-crime documentary or a genetics headline, and for good reason. It’s the case that proved genetic genealogy could crack decades-old cold cases where every other lead had gone dry. Here’s what the term actually means, how forensic science broke the case open, and what separates the DNA evidence that caught DeAngelo from the forensic evidence investigators had been sitting on for years.
Who Was the Golden State Killer?
Joseph DeAngelo was a 72-year-old ex-police officer and retired mechanic at the time of his arrest. He’d committed a series of unsolved murders, sexual assaults, and burglaries throughout California in the 1970s and 1980s, one of the longest documented crime sprees in American history. Investigators had DNA from the crime scenes for decades. What they didn’t have was a name to match it to.
That changed when investigators used crime-scene DNA to create a genetic profile that could be searched through GEDmatch. On this public genealogy website, users voluntarily upload their own genetic data to find relatives. The search produced distant genetic relatives, including a probable fourth-cousin match. From there, investigators built out the family tree, cross-referenced ages, locations, and physical descriptions, and landed on DeAngelo.
Forensic Evidence vs. DNA Evidence: What’s the Difference?
Both terms get used interchangeably in true-crime coverage, but they’re not the same thing. Forensic evidence is the broader category: fingerprints, footprints, trace chemicals, dental records, drug paraphernalia, firearms, anything physical that ties a person to a scene. DNA evidence is one specific type of forensic evidence, limited to genetic material recovered from body fluids such as blood, semen, and saliva, as well as from hair, bone, and skin cells.
Investigators collect DNA from both the scene and from suspects to reconstruct what happened and to build a case a court will accept. It’s especially critical in rape and sexual assault cases, where physical evidence can outweigh conflicting witness accounts. Genetic material shows up in places you wouldn’t expect, on clothing, on doorframes, on a discarded coffee cup, and DNA turning up somewhere it shouldn’t be is often the detail that tells investigators to dig further.
Want the deeper breakdown of how these evidence types are processed and used in court? A degree in forensic science covers the full range, from crime-scene collection to lab analysis to expert testimony.
How DNA Profiling Actually Identified DeAngelo
Here’s the part that makes this case a genuine turning point in forensic science: DeAngelo never uploaded his own DNA anywhere. Distant genetic relatives had voluntarily uploaded their DNA data to GEDmatch for unrelated reasons. Those uploads gave investigators enough of a family tree to work from, though it took additional investigation, not the genealogy match alone, to confirm DeAngelo as the source.
Investigators surveilled DeAngelo and collected discarded DNA, which directly matched the crime-scene profile. Officials arrested him at his home in Citrus Heights, California, near Sacramento.
In June 2020, DeAngelo pleaded guilty to 13 murders and 13 kidnapping charges and admitted dozens of additional crimes. He was sentenced that August to life in prison without the possibility of parole.
The case spread quickly through law enforcement circles for a simple reason: genetic genealogy doesn’t require the suspect’s DNA to be in a criminal database. It only requires a relative’s DNA in a consumer one, and by 2018, millions of people had already uploaded theirs.
Frequently Asked Questions
What does GSK stand for?
GSK stands for Golden State Killer, the name eventually used for the serial offender responsible for a string of murders, rapes, and burglaries across California from 1974 to 1986. He was identified in 2018 as Joseph DeAngelo.
How was the Golden State Killer caught?
Investigators used crime-scene DNA to search GEDmatch, a public genealogy database, which produced distant genetic relatives, including a probable fourth-cousin match. From there, they narrowed the family tree to DeAngelo, then surveilled him and collected discarded DNA that directly matched the decades-old crime-scene evidence.
Is DNA evidence the same as forensic evidence?
No. Forensic evidence is the broader category and includes fingerprints, footprints, trace chemicals, and firearms. DNA evidence is one type of forensic evidence, specifically limited to genetic material recovered from body fluids or tissues.
Why did the Golden State Killer case take so long to solve?
Investigators had crime-scene DNA but no matching offender profile in conventional law-enforcement DNA databases. The case only broke once distant genetic relatives’ voluntary uploads to a consumer genealogy site gave investigators a family tree to work from, which additional investigation then narrowed to DeAngelo.
Key Takeaways
- GSK means Golden State Killer – the name eventually used for the serial offender confirmed in 2018 as Joseph DeAngelo.
- Genetic genealogy generated the leads, not the identification – distant relatives’ voluntary GEDmatch uploads narrowed the family tree, and DeAngelo’s own discarded DNA confirmed the match.
- Forensic evidence and DNA evidence aren’t interchangeable terms – DNA evidence is one category within the broader field of forensic evidence.
- The case ended in a guilty plea, not just an arrest – DeAngelo pleaded guilty in June 2020 to 13 murders and 13 kidnapping charges and was sentenced that August to life without parole.
If cases like this are what’s pulling you toward the field, forensic science sits at the intersection of biology, chemistry, and criminal investigation. Read what a bachelor’s degree in forensic science actually covers before you decide which path to take.