Pat Docker lived quietly, as did most people she knew. If things could be dull or hard, at least there weren't too many miseries to contend with. Living at home with her parents and her four-year-old son Sandy, at 27 Langside Place, down in the red-bricked respectability of Battlefield in Glasgow's Southside, was enough.

On Thursday, the 22nd of February, 1968, 25-year-old Pat decided she was going to "the dancing", at the Majestic Ballroom in town. Dancing was just what you did in Glasgow back then. The Albert, the Majestic, the Locarno, the Plaza – each of the great halls catering to a different set of needs and expectations. But it was the Barrowland Ballroom, out east in the Gallowgate, that held the most notoriety and proved most popular. At some point on her journey to the Majestic, Pat changed her mind and headed there for reasons lost to the past.

Thursdays were different at the Barrowland: a "secret" over-25s night where discretion was crucial. Many of the men and women who attended weren't really there at all, at least not in their daytime apparitions. People introduced themselves under plausible pseudonyms, their wedding rings subtly slipped off at the door, ready for a night's worth of easily won intimacy – temporary relief from the slow oppressions of routine, work and home. As for Pat, people would sometimes ask about Sandy's father, but she was bored of discussing their "arrangement". Alex was living in Lincolnshire, still an RAF corporal. They weren't divorced yet, even if the question had been raised more than once.

It didn't take long upon arrival at the Barrowland for Pat to have a drink in hand, pick a spot and start surveying the crowd. She stood there watching, waiting, letting the band's tones ebb over her. There was a man standing close by, in his mid-twenties, though perhaps slightly older. He'd been poised there for some time. Handsome, she thought, with a shock of reddish or brown hair (it was too dark for certainty). They exchanged smiles, fumbled for polite small talk and took themselves onto the dance floor. For the next few hours, he would be her life.

The next morning was covered by a heavy frost. A joiner leaving for work noticed a figure lying on the ground by his lock-up on Carmichael Lane, just yards from Langside Place. Patricia Docker's body was found half-frozen between 7AM and 8AM. She had been dumped on Carmichael Lane at some point in the early hours, two streets over from her parents' home. The post-mortem established that she'd been strangled and almost certainly sexually assaulted. Her clothes and purse were missing, and tests by the coroner confirmed that she'd been on her period at the time of death. It made the evening's papers, though without too much noise: the fever and speculation would come later, jacked up to a storm's pitch.

We now know what the detectives then couldn't: Pat was the first victim of the serial killer later known as Bible John. Three young women were plucked from the Barrowland Ballroom between the night of the 22nd of February, 1968 and the 31st of October, 1969. Patricia Docker, Jemima McDonald and Helen Puttock. Each was married, each had been at the dancing, each had been spending the night away from their regular partner, each had been menstruating, each was strangled and sexually assaulted. The brutal shared idiosyncrasies of the crimes led police to quickly acknowledge the existence of a serial killer on the streets of Glasgow, at first in private, then publicly under the twin pressures of the press and public fear.

Patricia's body was found 50 years ago. Bible John has never been caught.

Instead, the intervening years have spawned a legend that has never quite lost its grip on the popular imagination of Glasgow, or Scotland as a whole. The killings provoked the country's largest ever manhunt, and countless words, suspects, books, documentaries, earnest speculation, pub theorising and bouts of urban myth-making. A British Zodiac killer and real-life whodunnit with no neat narrative conclusion, then or now.

The Barrowland Ballroom in Glasgow. Photo: William Arthur / Alamy Stock Photo

For some, it has come to mean more than just the fact of three dead young women: it's become a symbol of something wider than the sum of its own sad, savage parts. A remembrance of a violent time in a vanished Glasgow lurching into modernity, but still known for its inner-city tenement slums, marauding razor gangs, pockets of heavy industry and a murder rate far higher than its English neighbour. A vision of the city's past outlined in the Scottish writer Andrew O'Hagan's first book, 1994's The Missing. For others, the crimes simply left nothing else to say.

Bible John was different. These deaths felt like a judgment. They weren't the result of pub brawls, domestic disputes or localised gang warfare. These were crimes of a different order, committed somewhere beyond the commonly understood violence of their time and place. Crimes too difficult to categorise and too open to a limitless breadth of interpretation. Pure power and calculation. Randomness, you understood; these were anything but.

Some killings become legend, while others are forgotten as quickly as the act itself. Dr Adam Lynes is a Senior Lecturer in Criminology at Birmingham City University. "Really, it boils down to news value," he says over the phone; a sliding value that grows or shrinks "depending on how they kill, who they kill, how long the investigation ran before they were caught, or if they ever were. There has to be some sort of momentum."

The term "serial killer" takes its cues from the cliffhanger penny dreadfuls of the 19th century and the American comics of the mid-20th. "The murders follow a set pattern, just like Flash Gordon. The same format, repeated over and over again. It makes it easy for us to sustain our interest." It's illuminating, he tells me, having spent time researching "family annihilation" recently, just how little coverage there is on the subject. "You’d think that a father killing his whole family would be a national horror story, but they seldom are. That's because it's just one event. It's done, it's over. There's no mystery."

Bible John is pure mystery. The killings were a gift to the press. A ready-made sensation, stuffed with salacious detail and an almost boutique sense of revulsion. The moniker itself was a partial newspaper invention, leaning on the details of the third and final murder. Helen Puttock, aged 29. Her and her sister, Jeannie, had gone out at 8:30PM on Thursday, the 31st of October. 1969. First, to the Traders' Tavern on Kent Street for drinks and gossip, then to The Barrowland, just after 10PM. as the pub called for last orders.

Helen's husband, George, was understanding. He hadn't an issue with her going to the dancing chaperoned by her sister. Of course, you knew what went on there. And you knew all about Patricia Docker and Jemima McDonald – the 31 year-old who had been found by local children in a dilapidated tenement block in Mackeith Street, streets away from The Barrowland on the 16th of August, 1968 – but what could you do?

A 'Daily Record' poster appealing for information on Patricia Docker's death. Photo: Trinity Mirror / Mirrorpix / Alamy Stock Photo

Most of what happened next – and almost all of the available firsthand knowledge we have of the killer – we know from Jeannie's testimony to the police, taken over the course of the following days; endlessly repeated and amplified for many months and years after.

The girls split up soon after arriving at The Barrowland. A man approached Jeannie and they began the usual, awkward flirtation. His name was John, a builder from the distant southern suburb of Castlemilk. They had a dance and a drink together – then several more. Helen stood to the side, smoking and observing.

Before long, she had her own partner, a man of around 5'10" with reddish hair and a strange, slightly arcane elegance. He was a John as well, but so were most men. She didn't catch a surname. This wasn't unusual at the Barrowland, even if his single-breasted suit and leather half boots were. He was handsome, if sharp featured, with two slightly overlapping front teeth. You noticed his accent and its clipped, middle class inflections. Jeannie was later reported as saying that "he wasn't the Barrowlands type. Many of those who used to go were kind of rough or drunk. But he was nice, very nice, polite and well spoken."

Closing time arrived, with its fumble for coats and taxis. There had been a weird, fraught moment as Helen lost money in the cigarette machine, having pulled a switch for Embassy Blues. Her John had caused a scene, demanding a manager and laying into the establishment and its patrons. On the way out, he let the girls know that his father "thought these places dens of iniquity".

Religion flecked the conversation in the cab. Castlemilk John had thought better and hailed a night bus home: he has never been traced, despite the subsequent urgency of police appeals. Helen's John spoke of "adulterous women" and the age-old sectarian violence of Celtic and Rangers. To lighten things, Jeannie asked what he did for Hogmanay. He said he didn't drink, but prayed.

Things ran on, for some reason, to talk of foster children. John mentioned they were alright, citing Moses as an example (a possible reference Moses and the bulrushes from Exodus). The Old Testament allusions gave Jeannie an odd feeling. John insisted that Jeannie was dropped off first, to her house in Yoker, before retreating back to Helen's in Scotstoun, the West End suburb.

Helen was found yards from her home at 129 Earl Street the next morning. The same wounds, the same method – only, this time with a sanitary towel tucked under her arm, traces of semen left on her tights and a bite mark imprinted on her body. There was evidence of struggle: she had not submitted easily. The murder fell under the jurisdiction of the Marine Division, run by Superintendent Joe Beattie: a brilliant but inflexible detective. He knew what they were dealing with. Soon, a public appeal was carried on the front page of Scotland's papers:

[The killer] is thought to go by his Christian name of John. He may speak of having a strict upbringing and make references to the Bible. This man is quite well-spoken, probably with a Glasgow accent. There may be marks on his face and hands.

This was all the tabloids needed. Enterprising hacks soon conjured up Bible John as a moniker. It was a journalistic open goal: a weighty, almost comic bookish cloak for the terrible reality. It's an old trick. Bible John, the Yorkshire Ripper, the Suffolk Strangler. A film of unreality pulled over unthinkable acts. Soon, another layer was drawn over the case with the publication of the famous composite picture – a combination of Joe Beattie's urgings, Jeannie Williams' remembrances and the imaginative flair of a tutor at Glasgow School of Art.

The composite drawing of Bible John. Image: Trinity Mirror / Mirrorpix / Alamy Stock Photo

With its mocking eyes and air of tightly coiled, evangelical malice, the result looks more like a depiction of a devil than a sober investigative tool. It's something repeated – even more famously – in both the Zodiac and the recently solved Golden State case in California. The drawings feed into and become an essential part of the myth and suspense around the killings. Odd images, drawn from fragmented memories and pieced together under unspeakable pressure.

For some, Bible John meant opportunity. Murders often do. The chance to settle old scores and dole out suspicion without consequence. Of course, you could just get it wrong. It was dark, you'd had a drink. Maybe the ruffled, manic-looking man you saw at the bus stop had brown hair. It could have been blood that crusted his jacket sleeve. But how better to settle a historic grudge or flatten ancient disputes? Some instances are too bizarre to be fully forgotten, like the businessman who hired a team of private investigators to try and implicate an old school friend who had moved to Holland.

People thought they saw Bible John everywhere; in pubs, trains and sloping down suburban high streets.The killer could be lurking in barracks, subway stations or flats in Glasgow satellite towns. It didn't really matter. Things got so heavy that police were soon handing out cards to men in the city certifying that they weren't Bible John.

The search scaled, fast: 920 doctors and dentists were interviewed on the back of Jeannie's remembrance of the two overlapping teeth. Over 400 barbers and hairdressers quizzed on the hope of finding the distinctive reddish hair; 260 tailors who might have sold the odd, slightly continental suit. All of this and nothing back. Jeannie also remembered John bragging about a hole-in-one, so every golf club in the country submitted a list of those who had shot an ace. In all, 5,000 suspects were questioned before gradual elimination to zero in the first year of the inquiry alone. But Bible John disappeared after the murder of Helen Puttock, banked straight into the myth kitty of popular imagination.

Joe Beattie. Photo: Trinity Mirror / Mirrorpix / Alamy Stock Photo

Half a century has now passed. People have moved on or grown old. Some have forgotten and many don't really wish to remember, while Jeannie remained the only confirmed eyewitness until her death in 2010. Joe Beattie died in 2000, haunted and embittered by the case until the very end. I speak with several Glaswegians who were young people at the time of the murders. They each recall the atmosphere of hysteria, the heavy, overstimulated mood that clung to everything surrounding the case. It wasn't something you could close your eyes to, or absent yourself from.

My questions lead me to someone who says they know one of the victims' families, and wonders if I'd like to speak with them. But you wonder, who's benefit would it be for? To rip open old griefs, concerning mothers and grandmothers they never knew, to serve an anniversary they have no wish to commemorate. It makes me think of something picked up in my reading, from a 1997 book by two dogged tabloid journalists from the Scottish Sunday Mail. Bible John: Hunt for a Killer contains a postscript with some words from Helen Puttock's son David, then 28 years-old.

He never really knew his mother. "I would have killed the murderer," he told the two journalists. "I wanted him to go through the same pain mum did. The killer on the loose and I was doing nothing about it. I find that hard to cope with. He didn't exist: I can't let her memory go."

But there are others that it would be an oversight, not a kindness, to leave out. Joe Jackson was then a young detective in a north Glasgow murder squad when the call came in about the death of Patricia Docker. As the case grew and morphed, he worked it diligently and not without frustration. Joe's career rose over the decades, until he achieved the rank of Detective Superintendent before taking retirement in the 1990s.

It takes a week or so to get through to him on the phone. He's polite but firm. Frankly, he's sick of talking about the subject. So many years of the same questions, the same thoughts drudged up – there's nothing to add, nothing new to be uncovered. But as we speak, something thaws. Alright, how does 10AM Friday sound, he says. Here's the address, there'll be a coffee waiting for you, try not to be too late.

No one was beyond scrutiny or above an accusation, he says as we sit in the garden on an already sweltering summer morning. You wondered about all the handsome young detectives suddenly assigned to the case. Surely Joe Beattie didn't mean to hint at them too. The lads with a reddish tinge to their hair, the bright-eyed young, polite young men with the chiselled or neurotically angled noses.

You could feel his gaze on them, sizing them up the similarities, squinting hard for guilt. They went dancing too. Paranoia, perhaps. But with the killer uncaught, you could never be sure. There was mania in the air and the passing of time was making things more desperate, not less. Soon, there were psychics and mindreaders enlisted down at the Marine Division, all sorts of paranormal quacks to fill the void that normal procedures hadn't, or seemingly couldn't. The then-internationally-famous Dutch clairvoyant Gerard Croiset even submitted a dossier as a gesture of good will.

Joe Beattie thought in certainties and obsessions. He wasn't a man you crossed or came to with bad news, especially as a junior detective. And he was sure about Bible John. He'd know the second he laid eyes on him. No one worked harder than Joe Beattie. No one else lived the killings like him, aside from perhaps the victims' families. He had absolute faith in Jeannie, as a witness and investigative trump card. "He once said that he'd know him by sight, and know which pocket he carried his change in," Joe Jackson says disbelievingly. That was absolute faith. But it takes more than faith to solve a murder.

There are still regrets, says Jackson. He can't shake the sense that the investigation was doomed from the start. Though he talks about Joe Beattie with immense admiration, it's not uncritical. You could construe his ubiquity as "trying to make a name off the case", he says. There’s no doubt he was a brilliant detective, but mistakes were made. The procedures not followed and reliance on the press. "I hope you don't mind me saying this," he says early on in our discussion, "but they can be a nuisance as well as a help."

He recalls the "Bible John dancing squad", a team of 16 detectives brought to mingle in the Barrowland on the off-chance that the killer would return. "It might sound daft, but perhaps it helped. Even if the press wrote about it and the killer would have known, our presence helped kept people safe." But when we start talking about the case as an open-ended mystery, Joe stops me. "I'm sure he’s been caught. You know the man I’m referring to."

The criminologists of today would profile Bible John as a sexual sadist and a killer of organisation and precision, as indeed several have. Peter Tobin was – and remains – both of these things. In May of 2007, he was convicted of the brutal murder of 23-year-old Polish student Angelika Kluk. The crime appalled the country with a similar force to the 1960s Barrowland murders.

Kluk's body had been discovered a year previously under the floorboards of St Patrick’s Roman Catholic church in the Anderston area of Glasgow, not all that far from where Helen Puttock was discovered in 1969. She had been bludgeoned, raped and stabbed with a frenzy that shocked even hardened forensics officers and homicide detectives.

The 60-year-old Tobin had been working at the church as a handyman under an assumed name, Pat McLaughlin. By all accounts he was mild-mannered and dutiful, always hanging just to the side of things, always polite and well spoken. An urgent police appeal was raised in the days following Angelika's murder, on the 24th of September, 2006. He had been the last person to see her alive and was suddenly nowhere to be seen. A youth worker who knew "Pat" gave a photo to the police, who duly provided it to the television station. Five minutes after airing, the phones were jammed. They knew the man, but not the name. That was Peter Tobin, a figure of immense danger.

A few days later he was found, having fled to London, checked into a hospital with a fictitious complaint. A nurse recognised the face and alerted police. Forensic evidence soon proved his guilt of Angelika's killing, but the horrors didn’t stop unfurling. Tobin had been hiding at the church as a fugitive, on the run from an arrest warrant served in 2005 after he’d absconded from Paisley, breaking the terms of his place on the Violent and Sex Offenders register; the legacy of a 14-year sentence handed down in 1994 for a double rape and assault in Havant, Hampshire, committed in the flat he'd been living in with his four-year-old son.

As Joe explains, it's very rare for a killer to start in his sixties. He isn't the only one to make the point. Professor David Wilson is one of the best known criminologists in the country. He was one of the first to notice the parallels between Tobin and the fragments of fact we know of Bible John.

The strength of the facial similarities between a young Tobin and the famous composite, the obsession with religion, the sexual deviance and violence (all three of Tobin's former wives reported the appalling ordeals of their marriage and his sociopath's charm), the slightly crooked front teeth. He had grown up in Glasgow, was of age and reported as a dancehall regular, particularly at The Barrowland, where he met his first wife. The killings ended the same year that he left the city for a new life in Brighton.

"He was already a known figure on the system," says Joe Jackson, outlining how Tobin had spent time in Borstal in the 1950s for theft and forgery. Yet, somehow, he slipped through the net somewhere in the hysteria and the scale of the 1960s search for Bible John.

The detective and the professor's hunch was proved right, with horrifying accuracy. Further searches of Tobin's previous homes in Margate and Bathgate revealed the remains of two more bodies. Vicky Hamilton and Dinah McNicol, two vulnerable young women who had been reported missing in 1991. There's no need to recount the particularities of their deaths.

Tobin is currently incarcerated at Saughton Prison in Edinburgh, having been served a whole life tariff in 2008. The judge at his first trial, Lord Menzies, described him as an "evil man" – a judgment that no one has thought fit to appeal. Yet Tobin has never been charged for the Bible John murders, and any possible connection would likely never be proven, with the original DNA evidence taken from Helen Puttock too time-worn to be of much use. Detectives speculate Tobin could be responsible for a further nine murders. He has reportedly boasted of dozens more in prison, though won't concede to any officially. "I couldn't give a fuck [about the families]," were his video-recorded words.

Peter tobin (centre) in 2007. Photo: Trinity Mirror / Mirrorpix / Alamy Stock Photo

After all those years of searching, and this is what you think you've found. A frail old man in a flea-bitten cable knit jumper. Half a century of nightmares transmuted to the greying, squashed figure of a former part-time church handyman with a dodgy heart. How can it be anything other than a disappointment? When detectives in California discovered their great bogeyman, The Golden State Killer, earlier this year, it was the elderly Joseph James DeAngelo living as a "totally average Joe, an average person" with his granddaughter and daughter in Oakland – the truth somehow always smaller than the telling.

Officially, the case remains open. I contact Police Scotland and receive an assurance that "the murders of Helen Puttock, Jemima McDonald and Patricia Docker remain unresolved, however, as with all unresolved cases, they are subject to review and any new information about their deaths will be investigated".

Privately, I'm told it's doubtful that any serving officer would want to put their name to a piece on a still fraught subject. It's all just so long ago now. Most of the backdrop and context has receded into the past: the dancehalls, the victims, even the very physical texture of the city where it all took place. And if it is Tobin, that's it: the legend over, case just about solved. Recent reports talk of a frightened, cell-bound man who suffered a stroke in 2017. His capacity for evil is history now too, even if the repercussions of his past will always remain in the lives of those he affected.