How investigators built a genetic genealogy leading to 'Golden State Killer' arrest

20/20 EXTRA: Genetic genealogist and ABC News Consultant CeCe Moore showed "20/20" how investigators may have built a family tree to identify a suspect.
5:27 | 05/04/18

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Transcript for How investigators built a genetic genealogy leading to 'Golden State Killer' arrest
Traditional geologists are just yelling at people. Long ago hundred years 200 years ago. But somebody who's doing this type of work has to learn to put those families together coming to the present day. Because that's where you can find your suspect. Where the biological thing. I. Why did investigators choose Jett back it's open source and it's a public database the largest databases only accepts the life. So yes it and that to create quite a bit of saliva to you can't get that from a crime scene biological son. So investigators even if they wanted to dated have the ability to submit a sample on something like. Ancestry or 23 of me that's correct and those companies have purposely tried to make that difficult because they. They don't want on force you've seen their databases for these purposes. Investigators. Plucked it indigent imagine they are able to find some third fourth and even more distant cousins. How do you narrow it down from there. So the DNA alone is not going to be helpful it's only helpful in conjunction with the the family trees of those people sharing and day with us and so they have to look. At their pedigree here they're family trees and those are usually not jet. Jet Max is not a family tree website. It is a DNA website it's you have to take that information from Jack and then build family trees and that kind. You could get. Probably a lot of people trains to build the damage. That you need skill genetic geologists. To know what they're looking on jet action there's. Shoes. So here's your match. Wright who is this person. Doesn't help us must mean really. Cell bill that parents build their grandparents. And force that's a great. If we had a second cousin he could stop here but they only have third has so we have to go back. All the way to second. Rate. Grandparents have to treat continues to grow and it's just. Expand and expand its hands a third cousin on average shares less than 1% immunity. That means you should hear that second degree. You're gonna have to build this person's tree way are back. Couple hundred years most likely to get too that eight sets. Secondly. You're fortunate to have a lot of good supporting matches he can narrow it down just one but if you don't. You got a check out our. From 13 cousin how much would that narrowed the field you probably half thousands. So the next step is built forward. From these sets of sacking and parents and see. How many living people at least people who might fit the profile. And how to we view that well. It's brace all of us. Your and a half. Hundreds. Or thousands. Of people. Who dissent from the couple. And if you have no other matches you're gonna have to start looking at all of these descendants let's say this is balanced say it was 1000. How may have been of adult age at the time bars. Then you can go through its are eliminating people based on their page where they live thing. Like that that's and is Europe is down Britain and narrow. So say gets down to twenty. Now he only got when he lived in California the right time of the right age maybe it's some of the characteristics. That. Witnesses shared. You're and much sense. So this. TD game changer because every lot enforcement department. In the country there may be in the world. I just realized the power of genetic genealogy do you think the success of the Golden State killer case shall send a rush of law enforcement. On to these genealogy web sites. I do I don't know why that would happen. That's probably not something that's gonna have a lot of cases solved immediately if law enforcement is just uploading files there and thinking agony yet. Family trees that's not how it works it's much more in depth work and it takes a lot scale.

This transcript has been automatically generated and may not be 100% accurate.

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