Teenagers Pauline Parker and Juliet Hulme murdered Pauline's mother because they couldn't bear to be separated.Now, 57 years later, the pair have been found living just a few hours away from each other.
New Zealand lawyer Peter Graham's painstaking research into the 60-year-old murder has uncovered more information, including the discovery that Pauline Parker, now known as Hilary Nathan, is living in Scotland's orkney Islands.
Her former partner in crime, Juliet Hulme, who's now the successful crime writer Anne Perry, lives near the town of Portmahomack, also in Scotland.
In 1954, the inseparable pair, who were rumoured to be lovers, lured Parker mother Honorah Parker (45) into a secluded Christchurch park, where she (16) and Perry (15) took turns at brutally clubbing her to death.
Peter Graham, whose book So Brilliantly Clever has hit the shelves, has been fascinated with the murder for a long time.
As a young lawyer, one of his colleagues was Brian ocLelland, who had worked on the case.
"In the 1970s people in Christchurch still talked about the case," Peter recalls. "It was the most extraordinary murder."
Peter says his book is the first truly comprehensive review of what happened.
He tried to interview Perry (73), but was met with hostility.
"She said, 'Do you have any idea how unbearably painful this is for me? Can you remember what you did 50 years ago?' I said, 'Yes I can, actually,'" Peter recalls.
"I said I wanted to be as fair and reasonable as possible and she said, 'Fair! How can you be fair? You don't know anything about it,'" he says.
"I thought that was a bit unreasonable because the very thing I wanted was her point of view.
"She ended up saying, 'I wish to have nothing whatsoever to do with you or your book.'"
Since then, Peter has found several interviews Perry has given over the years about the killing.
In the book he accuses her of exploiting her past to sell her crime novels.
"Whenever a new book came out she gave interviews," he says.
Her former best friend, Nathan, remained undercover until an article by Chris Cooke appeared in New Zealand Woman's Weekly in 1997, revealing she was living in Kent in the UK, where she ran a riding school.
Once she was found, Nathan decided it was time to move on, settling in the Orkney Islands, which Peter describes as "about as far away as you can get".
Peter chose to leave Nathan (73) alone, since, unlike Perry, she hasn't courted publicity.
He concludes that, despite being inseparable as teenagers, the pair have never been in contact since.
His opinion is supported by the response Chris Cooke received when he spoke to Perry recently.
"one of the things he put to her was, 'Do you know Pauline is living just up the road?'
"She said, 'No! Why should I? I have no interest whatsoever in Pauline.'"
Peter's book attempts to unravel the possible motives for the murder.
It was claimed Honorah was seen as the main obstacle stopping Parker from going with Hulme and her family to South Africa, where they were to start a new life.
Peter says this claim never made sense to him.
"I could never understand, if they killed Mrs Parker, how that was going to enable Pauline to go with Juliet to South Africa."
However, his research revealed Perry's parents encouraged the girls to think Parker would be allowed to go, knowing that Parker's parents wouldn't let her leave.
"The Hulmes were making Juliet think they were doing all they could to keep them together, knowing it wasn't going to happen," he says, despite Perry's parents also wanting to separate the girls.
"It put Pauline's mother in the position of looking like she was the only opposition to the two girls going away."
Peter says, once he learned this, the claim made sense.
"In their warped minds they thought they were superbeings, above the law," he explains.
"And to them it was a solution to a problem."
During the girls' intense friendship they invented a new religion and imagined a parallel dimension, which they called "The Fourth World".
Interestingly, neither Nathan or Perry have ever married. Nathan is a devout Catholic, who spends much of her time in prayer. Perry has remained single, but says she has had boyfriends in the past.