The suspected Golden State Killer watched his sister get raped by two men when he was a boy, according to relatives, who suggested the incident could have contributed to his sick sexual assault and murder spree.
Joseph James DeAngelo, 72, who was named by police last month as the man responsible for at least 12 murders and 51 rapes in 1970s and 80s California, was "9 or 10" when his seven-year-old sister Constance was attacked on a German air force base, reports Daily Mail.
"Maybe that was the start of Joe going wacko," Jesse Ryland, 35, one of Constance's sons, told Buzzfeed News.
Professor Ann Wolbert Burgess, a psychiatric nurse who has examined the early years of serial killers for the FBI, said witnessing the rape could have created a vile fantasy that DeAngelo sought to recreate.
"Of course that would be significant and could have set the nucleus of the fantasy," she said. "What probably happened was that it was something that he kept on his mind."
As a child, DeAngelo moved frequently with his three siblings a child because his father, Joseph DeAngelo Sr., served with the US Air Force.
DeAngelo Sr. physically assaulted his wife, Kathleen, and was warned by cops that he would be thrown out of the Air Force if he hit her again.
Ryland said DeAngelo Sr. also hit the children, adding: "I'm pretty positive they were all abused like that."
And when Constance mentioned she had been raped, she and DeAngelo Jr were ordered by their parents never to discuss it, according to his nephew.
Retired cold case investigator Paul Holes, who spent 24 years hunting the Golden State Killer, said studies show serial killers often grow up in abusive environments.
But as many people who grow up with trauma do not become violent criminals, the impact of such an upbringing remains a "mystery", Holes added.
Constance's former wife, Kenneth Ryland Snr, told Buzzfeed she never discussed the rape during their marriage and he only heard about it from Jesse.
DeAngelo Sr. divorced his wife and moved to South Korea, where he had another set of children with the exact same names as those he had with his previous wife.
DeAngelo Jr. was previously charged with eight counts of murder in Sacramento, Ventura and Orange counties, but had a further four added this week.
Police tracked him down by comparing DNA found at crime scenes to profiles on public genealogy websites. These identified DeAngelo's relatives, who police eventually traced to him.
The latest charges relate to the 1981 killings of Cheri Domingo and Greg Sanchez, and the 1979 slayings of Robert Offerman and Debra Manning.
On December 30, 1979, Dr Robert Offerman, 44, and Alexandria Manning, 35, were killed at a home in Goleta near Santa Barbara.
Offerman, was an osteopathic surgeon, and Manning, a clinical psychologist.
The couple had their hands bound with twine when their bodies were discovered.
On July 27, 1981, Cheri Domingo, 35, and Gregory Sanchez, 27, were house sitting in Goleta when they were murdered in bed.
Domingo was found with her hands tied and suffering massive head injuries. Sanchez was shot and bludgeoned.
He was a former police officer in Auburn, California, but was terminated in 1979 for shoplifting a hammer and dog repellent from a drug store.
The Golden State Killer name was coined after the criminal began his reign of terror in the summer of 1976 with a series of rapes and burglaries in the cities of Carmichael and Rancho Cordova.
According to investigators, the masked killer would break into homes at night and tie up the victims – raping the female one if present – and subsequently pillage the homes of money, jewelry, identification cards and other valuables.
Some victims even told authorities that the Golden State Killer would call them up in the aftermath of the crime.
The serial killer's crime spree took a deadly turn in early 1978 when he is thought to have shifted his attention from burglarizing homes to murder.
The first victims of the Golden State Killer are believed to be Brian and Katie Maggiore of Rancho Cordova, who were murdered while out walking their dog on February 2, 1978.
The initial murders were followed by a series of additional murders and rapes in Modesto, Davis, Stockton and other San Francisco Bay area communities.
His final crime, believed to be the murder of an 18-year old woman in the Southern California city of Irvine, was committed in May 1986.