(CNN) Forty-two years.
That's how long it took police to have a breakthrough in the case of the Golden State Killer, the masked gunman who terrorized California in the 1970s and '80s with a series of killings, rapes and assaults.
His crime spree began in 1976 and went on for 10 years as the serial killer -- also known as the East Area Rapist and Original Night Stalker -- preyed on communities from Sacramento to Orange County.
Read more: The possible crimes linked to the alleged Golden State Killer
Police were not able to say they had a suspect -- until April 24, 2018, when authorities apprehended Joseph James DeAngelo, a 73-year-old former cop who police believe is the Golden State Killer.
But the jaw-dropping announcement didn't just bring an end to a decades-long investigation. It offered relief for the survivors, and it became an example of how police are using a new tool to crack cold cases: genetic genealogy.
Catch up on the Golden State Killer case one year after the arrest:
The Golden State Killer is believed to be linked to more than a dozen homicides and at least 50 rapes across multiple counties in California.
Police believe the case began in June 1976, when a woman in Citrus Heights, a city in Sacramento County, reported being bound and raped by a masked intruder. What followed were a series of attacks in and around Sacramento that police attributed to an "East Area Rapist."
At first, the reports were always of women who were alone or with their children. But by 1977, the list of victims had expanded to couples in their homes. And by 1978, the Golden State Killer's first known homicides occurred in Rancho Cordova, another city in Northern California. The victims, Brian and Katie Maggiore, may have witnessed him breaking into a home.
After the Rancho Cordova slayings, the killer started a second series of killings in the Santa Barbara area, more than 300 miles south of Sacramento. He haunted Southern California from 1979 to 1986, becoming known as the "Original Night Stalker."
Initially, investigators didn't see a connection between the attacks in Northern and Southern California -- but then a pattern began to emerge. For example, the serial killer would usually sneak into suburban homes at night, authorities said. If a couple was home, he would usually tie up the man, place dishes on his back and threaten to kill both victims if he heard the plates fall while he raped the woman.
"Over the years, we heard of homicides down in Southern California, and we thought it was the East Area Rapist," said Larry Crompton, retired detective for Contra Costa County Sheriff's Department. "But he would not leave fingerprints, so we could not prove, other than his M.O., that he was the same person. We did not know anything about DNA."
That changed in 2001, when DNA tests confirmed that the East Area Rapist and the Original Night Stalker were the same offender.
Then, in 2018, DNA led to DeAngelo's arrest. Authorities used a free genealogy and DNA database called GEDMatch to try to track down the killer who had evaded them for decades. They created a profile with crime-scene DNA, and in April 2018, DeAngelo's name emerged in what investigators believed was a pool of possible suspects.
Detectives then gathered DeAngelo's DNA -- some of it taken from the handle of his car door, some from a discarded tissue in his trash -- and found they had a match to evidence.
"It is fitting that today is National DNA Day," Sacramento District Attorney Anne Marie Schubert said the day after DeAngelo's arrest. "We found the needle in the haystack and it was right here in Sacramento."
DeAngelo faces 13 counts of murder with special circumstances, including murder committed during the course of a burglary and rape, as well as 13 counts of kidnapping for robbery.
He's charged in six counties -- Sacramento, Santa Barbara, Orange, Ventura, Tulare and Contra Costa -- and he will be tried on the multiple murder counts in a single trial in Sacramento. Prosecutors are seeking the death penalty in every county except Tulare and Contra Costa, which aren't eligible for that sentence.
In the wake of his arrest, DeAngelo did not enter a plea to the murder charges. His attorney, Diane Howard, described DeAngelo at the time as "depressed and ... fragile."
A Vietnam vet, DeAngelo became a police officer in 1973 and worked in the Northern California communities of Exeter and Auburn for six years before he was fired for shoplifting a can of dog repellent and a hammer from a drugstore.
DeAngelo later worked as a mechanic in Roseville, California, outside Sacramento.
He retired in 2017, and at the time of his arrest police found him in Citrus Heights -- the same neighborhood where the Golden State Killer raped the first of his known victims in June 1976.
Neighbors described him as reclusive, but not overly suspicious.
For years, Bruce Harrington pushed for California police to use DNA to catch the man who killed his brother and sister-in-law nearly 40 years ago.
"I began my quest in the mid '90s. It was 15 years until we heard that there was a DNA sample taken from our scene," he said at the 2018 press conference announcing DeAngelo's arrest.
In the early 2000s, he testified in front of the California Assembly in favor of expanding DNA collection by police and "pleading that they would embrace the power of DNA."
After learning that a suspect had been found, Harrington spoke to other survivors at the press conference.
"Sleep better tonight. He isn't coming through the window," Harrington said. "He's now in jail and he's history."