It’s been more than 40 years since a masked man unleashed a reign of terror across California that resulted in 12 murders and 45 rapes.

Dubbed the “East Area Rapist”, “the Original Night Stalker” and more recently the “Golden State Killer”, the unidentified serial killer has baffled the FBI since the late 1970s.

To this day, he has never been identified, let alone caught.

US authorities were first made aware of the rapist after his alleged first victim, known only as Jane, reported she had been raped one morning in 1976.

As CNN reports, Jane was attacked by a masked man on June 18 after her husband left for work.

She was lying in bed with her then three-year-old son next to her when an intruder stood in her bedroom doorway clutching a butcher’s knife and a flashlight.

File photo from the FBI's case into the 'Golden State Killer'. (FBI)

It was only after the masked man had bound, gagged and blindfolded her and her son and then moved the boy off the bed that Jane understood what the burglar was after.

Jane’s reporting of her rape triggered an FBI manhunt for the intruder. In the following decade, he would go on to commit 45 more rapes and kill 12 others.

“These cases are some of the most horrific I've had to investigate,” Erika Hutchcraft, an investigator for Orange County District Attorney's Office, says in HLN documentary series “ Unmasking a Murderer ”.

“They're not a one-time, you know, crime of passion, but these are almost passionless crimes. Very cold, very violent.”

File photo of sketches of the 'Golden State Killer'. (FBI)

Over the years similar murders were reported in Santa Barbara County in Southern California, yet while the “MOs” (modus operandi) appeared the same, there was little evidence and the technology of DNA was in its infancy.

It has only been since a documentary series and book are set to be released that the hunt for the serial killer has returned to public attention.

“He would not leave fingerprints, so we could not prove, other than his MO, that he was the same person. We did not know anything about DNA,” Contra Costa County Sherriff’s Department retired detective Larry Crompton said.

However, in 2001 authorities caught a break when DNA evidence revealed the suspect known as the “East Area Rapist” was also the “Original Night Stalker” and therefore also the “Golden State Killer”.

File photo from the FBI showing evidence bags and a face mask. (FBI)

If the killer is still alive, the FBI believe he would be in his mid to late 70s. He is described as a white male, close to six feet tall, with blond or light brown hair and an athletic build.

The FBI said detectives also have DNA from multiple crime scenes that can positively link – or eliminate – suspects. This will allow investigators to easily rule out innocent parties.

In 2016, the US intelligence agency offered a US$50,000 ($65,000) reward for information leading to the killer’s arrest.

File photo from the FBI of a crime scene linked to the 'Golden State Killer'. (FBI)