"He saw his sister be violated": The traumatic childhood of suspected Golden State Killer.

In April 2018, 72-year-old Joseph James DeAngelo was identified as the suspected Golden State Killer – the serial rapist and killer who terrorised California in the 1970s and 1980s, killing at least 12 people and sexually assaulting 51 others.

But decades before his prolific crime spree began, DeAngelo watched two men rape his younger sister, an incident which some say may have led DeAngelo to develop a preoccupation with rape.

DeAngelo was playing with his sister Constance in an abandoned warehouse on an Air Force base in Germany when two airmen walked in and raped her in front of him, Jesse Ryland, one of Constance’s sons told Buzzfeed News. 

“That’s pretty crazy for a kid to see his sister be violated… maybe that was the start of Joe going wacko,” said Ryland, 35.

It’s not just DeAngelo’s family that believes the incident may have sparked a fantasy with rape. Ann Wolbert Burgess, a psychiatric nursing professor who has examined serial killers’ backgrounds for the FBI said serial criminals commonly develop a preoccupation with their crime at an early age.

In this instance, it’s possible that DeAngelo, a former police officer, became preoccupied with rape after witnessing the sexual assault, Burgess added.

“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.”

Police hunted the Golden State Killer for four decades with little luck until they linked DNA from an old crime scene to DeAngelo using GEDmatch, a genealogy website.

Police arrested DeAngelo at his Sacramento home on April 24. He is set to return to court in Sacramento on Monday, however, it is not clear whether his sister's sexual assault will factor into the criminal trial.

Paul Holes, who spent more than two decades chasing the Golden State Killer wonders whether witnessing the assault was a traumatic or stimulating experience for DeAngelo.

"As more is found out about him and his childhood, we’ll get a better sense on how witnessing something like that would have contributed to him developing into the guy he was," Holes told BuzzFeed News.

Constance's son and DeAngelo's nephew, Jesse Ryland, said the siblings grew up in an abusive household, with DeAngelo's father often assaulting his mother.

Ryland also added that when his uncle and his mother told their parents about the incident in the warehouse, they were told never to discuss it.

Psychiatric nursing professor Burgess said this conflict with his parents may have been confusing for DeAngelo and may have affected his psychological state.

READ MORE: A 23-year-old woman woke to a man in her doorway. He was naked from the waist down.

Ryland told Buzzfeed News he never suspected his uncle was a violent man. He said he was a nice and seemingly normal guy.

However, when Ryland heard his uncle had been arrested, it made a lot of sense to him.

"Joe was young and saw my mom get raped," he said. "It instantly clicked in my head."

DeAngelo has been charged with all 12 murders attributed to the serial killer so far.

He is scheduled to appear in court on Monday.