Transcript for DNA led to arrest in cold-case murder of an 8-year-old girl in Indiana after 30 years
story. Tbi now has a more detailedtion of the suspected killer -- Thisuy is a monster, he's a coward. Rte the unthink about crime thataunted ft. Wayne, Indiana, for three de A big in a decades-ol cold case -- Reporter: Unt a clue hid no one genetic codanded a suspected child murdereeh . The abduction Dea of ye-old a chinsly has hated this community for over 30 years -- Reporter: The arrest part of a wef crime solving, king DNA th genealogy. To hunt the most violent offenders. 1988, a killer on theloose. Taunting po, ttening to strike again. The ff an innocent young thrust into the national spotlight, 8-year-old April tiy,o play at a friend'shouse, N making it me. Little apr kidnapped, raped,and strangled. Her body dumpen ditch. I thinkut April Oen I carried herture in my wallet for a long, long time. Reporter: The case bafflin authorities. The mysous killer inflicting more ar. Leaving rambling, misspeed messages, authorities say, like is caring on a barn door two yes after the murder. "I kill 8-ld ril, I will killagain." Then in , aorng police, more Nott at the homes of other young girls reading, "Hi, ho I've been watching you,ourey next victim How chilling are the details of this se I can assure you that anyone work on T case has been haunted B it. Match DNA found at the cri E, with a madman nowhere to be found. Police out this composite sketcased on H DNA profile. Ias par of this investigation when itstarted. And there has not been a day, a month, or a year that H gone byit someone in this room working on this case. Reporter: But fordecades, nothing. There's gotbe a bri ot it's goio come someday. And then there's moments you ha LI this is never going to beso. Reporte then on Sunday, a stunng development. 30 years. 30 years this family has Wai answers. And on Sunday, we WER able to give them some of thoseans. Reporter: Policeaking 59-year-old John d.ler into custody at this mobile Hom park where he lived. About sies from where April's body W found. I know he had a badtemper, always be cussing and swearing, throwinging stuff. Reporter: Investito say they aed Miller he knew why he was being quoned. His respond? "A Tinsley." Then, according T LI, a confession how cti is that confession in the case? The DNA still going to be the mostortant pie O evidence in this ca. You combine that with sion, and it's hard to figure out exactly what his defense is going to Oday is a big win for law enforcement. And this investigation illustras the dogge determine nation of the investigars who never ve N geup, and kept movingrd for the last 30 years. I C tell you how hpy I am Janet. She finallas closure to this. Life, every day. But I am spyhat she can finally rest a little bi knowing Thate finally caught. Reporter: Brk, police say, thanks to a new cutting-edge tool combining the poof DNA, geogy, a the inn crting a geneticfile from crime scene DNA, authoti they uploaded it to a public database called D tch, leading police toify relatives and zeroing in on suspected killer. John Miller. Investors laterrecovering a DNA sample F Miller trash. That, thesay, tied him to april'ser. Is there any concernut thegality of that? ING thrghme trash? The supreme court specificallyul on, can lice use evidence that they find in soe's garbage? And the awe is, typically, yes. If you're discarding your garbage outside home, you're prettyhiving up an expectationfvacy Ed wit trash. This is the has been announced in W were able to help investigators deine who suspect is. Reporter: Genetic genealogist CI Moore made a name acting families to long-lost tives. Thee genetic to reunite families, it's just as powerforrct. Ephe's working with a lab in Virginia andaw entempting to solve the most cng col cases like theey murder. We WER able to use G genealogy to narrow it down to two brother we get to that point, we turnt over to law ecement they have to investigate thoserothersnd determine which one is M likely the suspect. Reporter:ohn Miller brother, who did not want to be shown , says he tooas interrogated by police. They questioned me about an hour and a half. Plus they took my Reporter: He now says he S nothing do with his brother. R as I'm concerned, when Olde that he confessed to thiime -- my brother died. I'm done. Eporter:he murder of little April Tinsley, just T St in a series of high-profilecases. Cracked, investigators say, using the same cut g-ge technique. Tew DNA technology is just a won thing. Reporter: The first toe worldwide headlines, the so-c gn state killer, an elusive criminal whose reign of terror across California in the and '80s police included threatening phone calls to his victims. But in April, investigators say that match website HD lead the arrest of 72-yeaol Joseph deangelo,he former police officer now charged with aen murders, suspected of comtt more than 50 rape D just last month, according to police DNA left on restaurant napkin bng open the 1986 unsolv murder of 12-year-olde Welch, officers arresting66-year-d garyhartma what's pleaded not guilty. Today we' at aoint where if you're a criminal and left your DNA at the sc you might as well turn yourself in now. Reporter: Tda across the country in Pennsylvania, aop deejay arrested I a 25-year-old cold case.e murderf teacher Christie morack in 1992. Investigat using genetic geneogy to track 49-year-old Raymond Rowe, lifting a DNA sample from his water bottle and chewin G then charged with micide. Law enforcement agencies across the Coury are hoping to use theew technology to solve theircases.y work has always R aroundlies. Anrying to help provide closure. Reporter:orhe families of those murder victims,sh hope foowing decades of unanswered questions. Somewhere up there T's a little girl dancing with her anls she's brating. When you get on your knees GHT and talk to her in your drms and prayers, tell her. Th did it. They did it and we got him. Reporter: For "Nightline," I'm wit Johnson. T here, a lava bomb strikes a urine Hawaii.
This transcript has been automatically generated and may not be 100% accurate.