Transcript for New details on capture of 'Golden State Killer'
Back now with the latest on the golden state killer who terrorized California for decades. Investigators say they tracked him down using DNA in a genealogy website. Whit Johnson is in Sacramento with more. Good morning, whit. Reporter: Michael, good morning to you. We are outside the Sacramento county courthouse where later today 72-year-old Joseph Deangelo will make his first court appearance. Investigators telling us they used discarded DNA to help track him down. Genealogy websites and a long process of elimination sifting through family trees. This morning, new details of the rigorous investigation that detectives say brought down the golden state killer more than 40 years after his alleged killing spree began. Law enforcement sources telling ABC news they used a genealogy website to help connect Joseph Deangelo's DNA to past crime scenes. Taking Virginia evidence and then comparing it with family members within the online database until they found their suspect. Police say the 72-year-old appeared surprised when they swarmed his home Tuesday evening. No incident, he didn't say it wasn't me or anything like that. No. Really no conversation at all. Just the only thing he really said was that he had a roast in the oven. Reporter: His sister telling ABC news she could never imagine her brother carrying out the 12 murders some 50 rapes and dozens of burglaries he's accused of. It's kind of such a shock. You don't know how to react. Reporter: This as the portrait begins to form who taunted his victims with chilling phone calls like these. Gonna kill you. Gonna kill you. Reporter: Deangelo a Navy veteran serving in Vietnam, in 1973 then joining a police department around the same time the alleged crimes would begin. He outsmarted everybody for 40 years. Reporter: Farrell ward worked with him on the force. He kind of knew what the next step was going to be. Reporter: Neighbors say Deangelo was an average suburban homeowner except for his temper. He would yell so loud that you could hear him yelling from inside the house. A lot of times it would be cursing at us telling us expletive get out of here. Reporter: Elizabeth Hupp alleged the man she now believes is Deangelo tried to attack her in 1975 when she was just 16. Telling me if I screamed he would kill me. So we went out the back door. Reporter: She says her father witnessing the struggle scared him off but not before her attacker fired two shots killing him. I loved my father dearly and he took him away from me. Reporter: We're now hearing from the public defender representing Deangelo for the first time. She is accusing the sheriff's department of thwarting her efforts to even speak with her client and she's blasting the d.a.'s office for attempting to try this case in the media. George. Whit. Thanks. The book "I'll be gone in the dark" about the golden state killer was just released this year. Author Michelle Mcnamara died before it was completed by Billy Jansen helped her finish it with along with Brad Garrett. You said textbook. Textbook because serial killers can compartmentalize their life. They can kill, go home, make breakfast for the children, take them to school, go to work. So their life is like in boxes and he fits that profile. And the other thing you all saw early on that made you think he fit the profile was the idea of being a police officer or military. Absolutely. We knew he had a scanner because he was following the police patrols and he was hitting where the police weren't while they were staking him out so we thought he had a scanner. We didn't realize he had an official scanner. You say the police missed a big red flag early on. When he was accused of shoplifting, and what he was shoplifting was a hammer and dog repellent and he didn't want to have a trial or anything. He just sort of said, all right, I'm good. You can fire me. I'm fine. I'll walk away. He didn't want people to looking into what he was doing. Should have been a red flag a long time ago, Brad, but years and years later a genealogy site. Yeah, exactly, George. This is actually growing. Billy knows a lot about it. In the law enforcement community, it takes a fair amount of time, it can take a lot of money, particularly if you only have like a distant relative and really trying to track it through the system, but it does work. So like you say it takes a real commitment and the question now, it happened so long ago. Do you think they'll be able to tie him to other crimes. I think so. That's the big thing I'm looking at. Building a time line. Where was he and how many other crimes could he have been connected to. Brad, when we talk about the profile of serial killers you say he compartmentalizes life so for the last 15, 20 year, no crimes. I don't know. I think as Billy digs into this, law enforcement goes further, they're going to run DNA against other sexual assault cases in or around probably throughout California. So, you know, serial offenders tend to either die or get caught and so because, George, it's such an obsession for them so the real key is what happened between '86 and 2018. Michelle Mcnamara, of course, died before this crime could be solved. You tweeted that the key players in getting the book came together came together the night before the verdict. We had a great event this her hometown. Her family was there and I went to bed. I got a text message that there was a press conference and I just started trying to confirm it and find out what's going on. It was a surreal moment. It was while we were saying we are are going to catch this guy. You had idea the night before. No idea the night before it was happening. Yet it got done. Incredible work. Thanks for being here. Michael.
This transcript has been automatically generated and may not be 100% accurate.