Transcript for 'Golden State Killer' allegedly caught after almost 42 years
It was a power play. I'm both nowhere and everywhere. You've may not think I have something in common with your neighbor, but you do. Me. Reporter: Despite the discovery of a DNA link among his many vicious crimes, the identity of who might be the golden state killer would remain a mystery for decades. How frustrating was that at the time to not have a name or face to go with the DNA? It's frustrating. I arrived at the scene with my little suitcase. Pathologist Dr. Peter Speth investigated when DNA was in its earliest stages. I always had duplicates. That second kit sat in the coroner's possession for 38 years untouched. So the swabs collected from Charlene Smith's body were pristine. For years those pristine swabs languished in an evidence room past rows of case files T. DNA of a man who police say killed 12 people and raped 50 sat undisturbed. For almost four decades police were waiting for science to catch up. Little did they know years later a Jean olg flourish. C.C. Moore helped pioneer the use of DNA to build family trees. Give me a hug. Adopt tees and people with unknown parent age started coming to me to identify their parents. Law enforcement was slow to realize its power until detectives wondered if it could create a road map to a serial killer. I started listening to phds start to explain this and I thought it was interesting. He uploaded it on to a no frills Jean olg website. How did you place the DNA into the website. I created an undercover account and uploaded the golden state killer's profile and allowed them to do their magic and produce a list of people who shared DNA with my offender. Why did you choose general match? The largest databases only accept saliva samples. Remember investigators had the DNA from a rape kit to work with. Investigators didn't have the ability to submit a sample on ancestry? That's right. Companies want to make that difficult because they don't want law enforcement using their databases for these purposes. Ged match said they were not approached by law enforcement enforcement. Despite the database being smaller than 23 and me and ancestry.com detectives got lucky. I got a list of individuals that shared DNA with the offender on the order roughly a third or fourth cousin. Reporter: He spent months building family trees and working with teams of investigators. They poured over obituaries, grave sites, KREN -- census records to begin a process of elimination. At the end sifting through the family tree, how many people did you end up within your final group? We start looking at the geographic profile and seeing he has a Sacramento connection and a southern California connection. We're evaluating these people of the right age, somebody roughly 5'8", 180 pounds. We settled on five. How do you narrow in on Joseph Deangelo? We started focusing in on him because he looked better than the remainder. He wasn't a prime suspect, was he? No. He just rose to the level as many people have. It wasn't anybody thinking this is the guy. To narrow the focus police knew they needed to get fresh DNA from Deangelo to match against that 1980 sample. How did they get the DNA from him? By a surveillance team that watched him for days. When he went to a public location he discarded his DNA at that public location that was collected. Reporter: Paul Holtz retired in March. Where were you when you got the news? I was outside a restaurant in Colorado. I received a call from the Sacramento d.a.'s office. They said don't say a word. The DNA came back and it's looking like it's him.
This transcript has been automatically generated and may not be 100% accurate.