Here's a step-by-step explanation of how law enforcement used a public genealogy database to identify and apprehend Joseph James DeAngelo, known as the Golden State Killer:

Step 1: DNA Matching:

Investigators in the Golden State Killer case had access to DNA evidence from crime scenes, including semen and blood samples collected from several attacks. They uploaded the DNA profile of the Golden State Killer to a public genealogy database, which allows users to compare their DNA with others to find genetic relatives.

Step 2: Genetic Genealogy Research:

Law enforcement began searching for potential matches by looking for individuals in the database who shared a significant amount of DNA with the Golden State Killer. They utilized techniques such as genetic genealogy and haplotype analysis, which involves the examination of specific DNA segments shared by related individuals.

Step 3: Narrowing the Focus:

The initial search on the public genealogy database produced thousands of potential matches. Investigators narrowed down the list by focusing on individuals who:

- Lived in California or had connections to the areas where the crimes occurred

- Were of the appropriate age and had the physical characteristics fitting the suspect description

- Had genealogical trees consistent with the DNA profile

Step 4: Constructing a Family Tree:

Investigators worked with genetic genealogists to construct a family tree based on the DNA matches and public genealogy records. By carefully analyzing the tree and identifying common ancestors with the suspect, they were able to narrow down their search even further.

Step 5: Identifying the Suspect:

As they continued to trace the family tree, investigators noticed a pattern. Many individuals in the tree had a common great-great-grandfather named Joseph James DeAngelo. This led them to zero in on a distant relative of Joseph James DeAngelo, who became a key lead in the case.

Step 6: Obtaining DNA Sample and Confirmation:

Once they had identified the potential suspect, law enforcement obtained a DNA sample from a discarded item (e.g., trash bag) to compare with the DNA profile of the Golden State Killer. The comparison revealed that the DNA matched, conclusively identifying Joseph James DeAngelo as the prime suspect.

Step 7: Arrest and Investigation:

On April 24, 2018, Joseph James DeAngelo was arrested at his Citrus Heights home. Subsequent investigation gathered more evidence, including additional DNA matches and a confession, which further connected DeAngelo to the crimes attributed to the Golden State Killer.

This innovative approach of using a public genealogy database was unprecedented in law enforcement investigations and played a crucial role in solving one of the most notorious serial killer cases in California's history.