The Washington Post reports:
If Joseph James DeAngelo Jr. is the Golden State Killer, as police allege, then he spent his 30s in a nearly nonstop frenzy of sadistic violence — breaking into a house every few weeks in the late 1970s, raping dozens of women, and escalating to serial killings before the crime spree suddenly ended in 1986.
And if DeAngelo is the killer — police say his DNA proves that he is — then he fit these 45 suspected rapes and 12 suspected murders into an astonishing double life. He was a Vietnam War veteran before the spree, a police officer during the spree, and then a husband, father and grandfather who lived quietly among the same communities that the Golden State Killer had terrorized.
Assuming they got the right person (and DNA is rarely wrong) this is an incredible achievement.
It seems how they did it was check the DNA at crime scenes with an genealogy company that has tens of thousands of DNA samples. He was not registered with the site, but a relative of his was. Once they got the DNA match to the relative, they then had a short list of a dozen suspects or so.
This may catch other criminals in future. They may never ever give a DNA sample themselves, but if even a fourth cousin does, that may be enough of a match to send the Police in the right direction.
Of course you would generally expect the genealogy company not to share your DNA, but some sites allow people to post their DNA publicly, and it seems this was one of those sites.
So we should be very thankful to the distant cousin whose interest in genealogy has led to this arrest.