"How does a middle-class housewife who has no enemies in the world just vanish into thin air?"
On 29 December 1969, a woman called Muriel McKay was abducted from her home in Wimbledon, southwest London. She was never seen alive again, her body never found.
Inside the house, the telephone had been ripped from the wall, the contents of her handbag were scattered about, and a roll of twine and a billhook – a gardening tool with a thick blade – were found discarded.
The disappearance became the first high-profile kidnap-for-ransom in the UK, a story that dominated news headlines in the days that followed as it emerged the criminals were demanding £1m – what would be about £15 – £20m today – for her safe return.
Mrs McKay was a 55-year-old housewife, a churchgoer, a loving mother to her three grown-up children. She was married to Alick McKay, deputy at News Limited to Rupert Murdoch, the Australian media tycoon who had made headlines relaunching The Sun newspaper just a month before.
It was a case of mistaken identity; Murdoch's wife Anna the intended target. Nevertheless, the kidnappers persisted, making ransom calls, identifying themselves as M3, and writing letters to the family demanding the money.
After the police investigation led officers to a farm owned by Trinidad-born Arthur Hosein, he and his younger brother, Nizamodeen, were charged and later convicted of Mrs McKay's kidnap and murder – despite the fact police never found her body.
More than 50 years later, it is a story that still has many unanswered questions, says documentary maker Joanna Bartholomew, who grew up in Wimbledon and has spent almost three years investigating the case.
"It's a story that has been slightly forgotten, but it was a huge, huge case in its time," she tells Sky News. "It's a very mysterious case, there's so many odd things about it. [The kidnappers] completely got the wrong person, they weren't professional, Rupert Murdoch was involved.
"This woman just vanished. And we'd never had a kidnapping before in Britain, not for ransom. This was a crime you might see in America or possibly in Italy. Everyone kept saying this isn't a British crime, we don't have kidnapping in this country, particularly for ransom of a million pounds."
Bartholomew has produced a new documentary, The Wimbledon Kidnapping, about Mrs McKay's disappearance. In the programme, her daughters speak for the first time – as does Nizamodeen Hosein, who was deported back to the Caribbean following his release from prison after more than 20 years. Arthur Hosein died in prison in 2009.
The programme throws new light on a case that confounded police at the time. Taking place years before the discovery of DNA and mobile phone evidence, the documentary suggests there were flaws in the investigation made by detectives eager to convict, and amid tensions between the police and Windrush generation immigrant communities.
Bartholomew spoke to some police officers who covered the disappearance, who told her it is the case that has "haunted" them more than any other.
Mrs McKay's daughters, Jennifer and Diane, also feature in the programme, describing their desperate attempts to deal with ransom demands in the hope of saving their mother.
"The McKay family have never had any kind of closure – I hate that word, but they've never had answers to questions they would like to know," says Bartholomew. "Muriel McKay's children are now in their 70s, 80s, and as you get older you look back at your life and you sort of hope things that have troubled you may be resolved.
"I guess there was a feeling of trying to find something out that might help them end that chapter."
Following her disappearance, Mrs McKay's family watched as their mother reduced to a victim, a two-dimensional figure known only by the two or three photographs that had been released to the public, and the fact she was missing.
"Here was a woman who was a loving mother, she liked to travel, she entertained for her husband and his Fleet Street friends," says Bartholomew. "One of the things the family felt was that they wanted to reclaim that memory and say, no, she was much more than [a victim]."
The producer also wanted to find out about Hosein, and spent several days talking to him in the Caribbean.
Nizamodeen Hosein was 22, 12 years younger than his older brother, when he was convicted. The documentary paints a portrait of a man who was controlled by his sibling; Arthur Hosein's daughter is another person to speak out, and while she does not have good things to say about her father she is clear that she does not think her uncle is capable of murder.
There is evidence in the documentary that points to possibilities others may have been involved.
"Why would two people who are not professional criminals have such a crazily ambitious idea to kidnap Rupert Murdoch's wife?" says Bartholomew. "[Nizamodeen] had only been in the country for a few months. It's extraordinary."
Of meeting him, she says: "In some ways, he was what I expected and in some ways he wasn't. I think you have to make up your own mind about him. He's an old man now, and there was a sense with him, too, that he was trying to come to terms with what happened to his life.
"I think that's maybe why he agreed to talk for the first time ever, because he'd never talked before."
She adds: "He was obviously a very troubled man… when [you] meet people who are supposed to have done very evil things… it's not that you walk into a room and have this sense of, 'oh, my goodness, this man is evil'. It's not like that.
"They're just people who have done terrible things – or not, as the case may be."
The Wimbledon Kidnapping is out now on Sky Documentaries and NOW