How did a genealogy website help catch the Golden State Killer?
In 2018 police found a killer by building family trees from his distant cousins' DNA on a genealogy site.
Guess first
?
Which one is it?
They uploaded the killer's DNA profile to a genealogy website and let his distant relatives lead them to him. In April 2018, California authorities charged Joseph DeAngelo, then 72, with eight counts of first-degree murder on the basis of DNA evidence, after investigators had identified members of his family through forensic .
The website was , a public site where people upload DNA data files from genealogy testing companies to find relatives. It was founded in 2010 by Curtis Rogers and John Olson, mainly to help genealogists, including searching for birth parents. When investigators uploaded the killer's profile from a crime-scene sample, GEDmatch identified 10 to 20 people who shared with him. A team and a genealogist used the list to build a large family tree, which gave two suspects. A relative's DNA test ruled one out.
Then came old-fashioned detective work. Investigators built about 25 family trees, and the tree that linked to DeAngelo contained approximately 1,000 people. Using clues such as age, sex and place of residence, they ruled out suspects one by one until only DeAngelo remained. On 18 April 2018 a sample was collected from the door handle of his car, and another from a tissue in his garbage. Both matched samples from Golden State Killer crimes.
Step 1: Upload the profile
The crime-scene DNA goes to .
Step 2: Find distant relatives
10 to 20 people share with the killer.
Step 3: Build family trees
About 25 trees; the right one had around 1,000 people.
Step 4: Rule people out
Age, sex and residence narrow it to DeAngelo.
Step 5: Confirm with a sample
DNA from his car door handle and garbage matches.
DeAngelo pleaded guilty in June 2020 to 13 counts of first-degree murder and 13 of kidnapping, and received multiple consecutive life sentences without the possibility of parole.
Try it
How much DNA is shared
y = 1 · 0.5ˣ
Challenge · Set each step to keep half (0.5). What fraction do third-degree relatives, such as first cousins, share?
Quiz me
0/3
Recap
Your DNA is also your relatives' DNA, so a search that finds a cousin can lead to a suspect, and raises privacy questions for the whole family.
💡 A trick to remember it · A family tree is a map with a pin on the guilty branch: relatives' DNA tells you which branch to climb.
Surprising fact · The family tree that led to the Golden State Killer contained approximately 1,000 people, out of about 25 trees built.
Connects to
- 🗄️ How big are forensic DNA databases, and what is a “cold hit”?
- 🧬 How does a forensic DNA profile work, and what do labs actually read?
- ⚖️ What has DNA taught us about wrongful convictions and the Innocence Project?
- 🗂️ What is personal data, and how do companies make money from it?
- 🇪🇺 What rights does GDPR give you over your personal data?
- Familial dna searching
- Genetic genealogy
- Privacy
Sources (5)
No source, no claim. Every fact in this lesson (18 claims) cites at least one of these.