Growing up, Matthew Roberts had no idea he could be son of one of the most infamous mass murderers in history. He didn't even know he was adopted.

Raised by a Swedish mother and a German father, he was led to believe he was their biological child. His sister dropped a bombshell and told him he was adopted when he was in fifth grade. Shocked, he confronted his parents, who told him the truth - or at least, part of it.

"I just left it at that, as I really didn't have any desire to find out who my biological parents were, or to get in contact," Roberts tells news.com.au.

At the age of 25, Roberts became engaged and started the search for his biological parents at the behest of his fiancee Gina, who was curious as to what nationality any future children would be. Four years of dead ends, non-identifying information and bum steers followed, until Roberts finally got in contact with his birth mother.

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"I spoke to the woman who worked for the adoption agency at the time [of his adoption] and was still working there, and she tried to warn me about my mother: that she was a little mentally ill, I guess."

Roberts started mail correspondence with his mother. "She seemed perfectly normal to me, talking about cats and growing rhubarb in her garden, and seemed like just a hippie - but then it became pretty clear at a certain point that she did have some mental problems that progressively got worse as time went on, it seemed, or at least that she was unable to hide.

"Then I asked about my father."

Matthew Roberts appears to the spitting image of Charles Manson. Photo / Getty Images
Matthew Roberts appears to the spitting image of Charles Manson. Photo / Getty Images

Roberts' mother [who cannot be revealed for security reasons] was from the small Wisconsin town of Exland, which shared a post office with the neighbouring Eau Claire - home of Mary Brunner. Brunner had fallen under Charles Manson's spell, and accompanied him and a few other girls on a VW trip across the country to "gather attractive girls for their so-called family."

"Mary and my mother were friends and she introduced him to her", he explains. "They then drove back to Berkeley, and my conception happened somewhere in the interim. This was confirmed by him, in letters I have from him, addressing the issue.

"They hit it off right away," Roberts continues. "He was particularly fond of her, so much so that the other girls got jealous of my mother, and Charlie bought her a bus ticket to go home.

Charles Manson in court in 1986. Manson was the enigmatic cult leader who was found guilty of the conspiracy to commit the murders of seven people - most notably actress Sharon Tate. Photo / AP
Charles Manson in court in 1986. Manson was the enigmatic cult leader who was found guilty of the conspiracy to commit the murders of seven people - most notably actress Sharon Tate. Photo / AP

"Some of his letters seem to suggest that he knew she was pregnant, and that may have been another reason for sending her back."

Roberts learned the truth in stages. Initially he found out his mother had named him Lawrence Alexander, but that she could only tell him his real surname in person, for security reasons. The story slowly became darker.

"She said that I was the product of a rape - I was conceived in a drug induced sex orgy with multiple people involved, and that she was raped. Later she recanted, and said that she might have confused male aggression for male vigour, or male vigour for male aggression.

"She said she was part of a very infamous hippie group in the '60s that involved Charles Manson. I asked if he was at that orgy and she said, 'Yes', and I asked, 'Did you have sex with him?' and she said 'I don't know'.

"Well, when I looked in the mirror, I looked like his twin."

Charles Manson in prison in 2014. Photo / AP
Charles Manson in prison in 2014. Photo / AP

Whenever Roberts brought it up with his mother, she got angry and shut down any further conversation. Seeking more concrete answers, he contacted Manson.

"I wrote letters to him in 2001 and asked if he remembered my mother, and the events surrounding my conception and he wrote back immediately - like two weeks later, I got a letter from him and another one right away, and he remembered my mother and told me details that he seemingly could not have known had he not at least been there at that time: Details about her father - my grandfather - chasing him away, calling him 'bad biker trash outlaw bandit' - all of which was consistent with what my mother told me."

In fact, it was Roberts' grandfather scaring Manson off that saved her from a worse fate, especially considering how her fragile mental state might have made her susceptible to influence. "She was able to avoid all of the murder and mayhem, as she was not around for all of that."

Despite corresponding via post over the past fifteen years, Roberts has never met his father, and admits he doesn't have much of a relationship with Manson. As may be expected for the son of one of the 20th Century's most infamous figures, safety is an issue.

"At one point I asked him if I could come visit, and he was suggesting that bad people were intercepting my mail - murderers and rapists - that were threatening to do me harm once they got out of jail and he suggested that these people were not people I would want knowing who I was. So this, mind you, was Charles Manson telling me that these were bad people wishing to do me harm so I took it to heart and chose not to go visit him.

Three female defendants in the Manson court case are shown, from left to right: Susan Atkins, Patricia Krenwinkel, Leslie Van Houten. Photo / AP
Three female defendants in the Manson court case are shown, from left to right: Susan Atkins, Patricia Krenwinkel, Leslie Van Houten. Photo / AP

"One letter he wrote, 'I hope you weren't in St. Louis recently because there was a little black girl who was cut up into pieces and put into a brown suitcase and they're already calling you Suitcase Alexander [his given surname from his birth mother] because they want to get you accused of the crime to get you in jail."

Despite their mail correspondence, Roberts is under no illusions regarding their bond.

"Some of his letters are very heartfelt and kind of fatherly advice, and then others he gets mad at me like when he got upset about the lyrics of my song that reads, 'What would you do if you found out Charlie Manson raped your mum?'"

This was prior to his mother recanting this, and now Roberts realises the full impact of his public claim.

Charles Manson is escorted to his arraignment on conspiracy-murder charges in connection with the Sharon Tate murder case, 1969, Los Angeles. Photo / AP
Charles Manson is escorted to his arraignment on conspiracy-murder charges in connection with the Sharon Tate murder case, 1969, Los Angeles. Photo / AP

"They don't treat people in prison very well who are accused of rape, so I regret any problems that might have caused him. He also got upset when I called his girlfriend Star [Afton Elaine Burton] an opportunistic pariah, even though I was just kind of joking - but it became a headline and went viral.

"I would like to have a better relationship with him and I would like to visit one day before he passes."

And time could be running out, with reports this week that Manson's death is "imminent".

Not surprisingly, being the son of Charles Manson isn't really a door-opener when it comes to pursuing a life in music, Roberts chosen profession. Despite this, Roberts has enjoyed a successful career, with his music published in 52 countries. Still, there are those who think he is attempting to use his father's infamy to further his own agenda.

"I get a lot of flack from people saying I'm trying to ride on some kind of legacy or trying to build a career", he admits. "It has done nothing to advance my career; if anything it has ruined it.

"From the age 29 until this very day - I am now 48 - I live in chaos and uncertainty, and frankly it sucks. I think it is the worst possible outcome."

Cult leader Charles Manson, who is linked to the Sharon Tate murders, is seen 1969. Photo / AP
Cult leader Charles Manson, who is linked to the Sharon Tate murders, is seen 1969. Photo / AP

There are also many who doubt that he is Manson's son, despite both parents acknowledging this to be so - and the uncanny resemblance. Despite the trouble his lineage has caused him, the alternative would be much worse.

"It seems to me to be much more troubling a thought [if I were] to find out it's not true, because then I have to wonder how it was that I got led down this path in the first place. I'm a proud double-Aries alpha-male and I'm not happy at the thought of being a sucker. I'm certainly not trying to be anyone's follower.

"It's all very surreal to me; it's like a Hollywood movie. I haven't really personalised it much, or allowed myself to have an emotional connection to it ... any of it."

"It has been good, bad and ugly - but never boring."