One June afternoon in 2009, a thin man dressed in black boarded a bus to Sligo, a small coastal town not far from the Irish border . Three days later, after a quiet weekend spent largely alone, the man was dead – his passing the first act of a mystery that has now baffled and compelled police forces, journalists, film-makers and internet sleuths for over a decade.

On Sunday, in the early afternoon, he left the hotel for the town's only taxi rank and asked to be taken to a quiet beach, where he could swim. The driver took his softly-spoken fare to Rosses Point, the peninsula known for its dramatic views, about 15 minutes away by car. On arrival, Bergmann got out, surveyed the vast blue expanse and seemed satisfied with the choice. Instead of disembarking as expected, he took the taxi immediately back to Sligo, where he settled into the groove of another evening alone.

The next day passed without much incident. Bergmann made his way to the General Post Office at 10:49AM, where he bought eight stamps and some airmail stickers. He ran some errands around town and arrived back at the hotel to eat and take an occasional cigarette outside, keeping himself politely but firmly to himself.

The first hotel the man tried was full – it was a Friday night at the peak of the summer tourist season – but he had more luck at the Sligo City Hotel on Quay Street, where he paid for three nights upfront. Writing in the register, he put down his address as Ainstettersn 15, 4472, Vienna, Austria, which matched his Germanic accent. With the same pen he gave his name as Peter Bergmann. At no point was he asked for identification.

Arriving at 6:28PM, as the evening sun warmed the water of Sligo Bay, the man took a taxi to the centre of town. In the years since, some have offered this as proof of his unfamiliarity with Sligo: to walk from the station takes just over ten minutes at an ordinary pace. That said, he did also have two bags to carry, and his greying hair and slight frame suggested he could have done with the help.

The next morning, not long after 6AM, a local man and his son were jogging along the sand, amid the last remnants of a sea fog. They were the first to find the washed up body of a thin, middle-aged man with closely-cropped grey hair. Peter Bergmann was dead, but the mystery that has since surrounded his story was only just beginning.

Later, it was established that up to 16 people had seen Bergmann on the beach that afternoon. He wasn't trying to hide himself. They all remember a jovial, if formally-dressed, figure greeting the strangers who crossed his path.

Just after 1PM on Monday the 15th of June, Peter Bergmann checked out of the hotel and deposited his key at reception. He left one of his original bags – a purple plastic "bag for life" – and what appeared to be a new black luggage case. He took a circuitous route to the bus station; at one point he stopped in the doorway of a shopping precinct and waited, poised like a man about to turn back. Instead, he made his way to the bus station and, on arriving, read notes on scraps of paper he'd picked out of his pocket, before tearing them up and depositing them in a nearby bin. The bus to Rosses Point departed at 2:20PM.

I made my way straight to the Sligo City Hotel. I wanted to start there, just as Bergmann had, but I couldn't say exactly why. For the last year or so I have spent much of my working life covering cases of missing people in the UK and further afield. Their stories can sometimes tell us all sorts of things about the way we live now, about loneliness and pain. But they can often speak to nothing else than the missing person's own private mysteries.

I visited Sligo for the first time on a Friday in May of 2019. I'd flown to Dublin that morning and taken the three-hour cross country train, passing field after field, town after town, before arriving at Sligo's train station, directly above the bus station where Peter Bergmann arrived all those years ago. It was a fine day, so I let my feet direct me into town, past the full wall mural to WB Yeats, Sligo's most famous advocate.

On a Saturday afternoon in September of 2019 I met with Detective Inspector Ray Mulderrig at Sligo Garda station. He is the third DI who has had ultimate responsibility for the Bergmann case. In 2009 it was John O'Reilly, who has since been promoted and moved to a different district. Ray talked with precision, and politely corrected me when I asked if he was fascinated with Peter Bergmann. "We don't get fascinated in cases," he said. "They arrive to us and we deal with them."

Peter Bergmann's last days were pieced together by trawling through Sligo's CCTV network. The shuffling, scrupulously-careful figure had left the hotel each day armed with his purple plastic bag, full, and returned with it empty. The hours in between were a mystery. It appeared that he had deposited his belongings in various bins around the town, taking great care to avoid being picked up by surveillance cameras. Watching the snippets of available footage is an odd sensation, like watching a ghost move through the world of the living. The man who spent his last days as Peter Bergmann has never been identified.

As more days passed, the mystery mutated into something the police hadn't really encountered before. Missing people were one thing; this was becoming something else entirely, almost supernatural in its intrigue.

It quickly became apparent that there was something strange about Peter Bergmann. The total lack of ID or belongings, and the fact that the labels of his clothes had been crudely hacked away with scissors. Authorities checked his address and found a vacant lot in Austria, while extensive searches didn't reveal any "Peter Bergmann" who could possibly match the man's description. The letters he posted from Sligo have never been traced.

There are all sorts of ways we deal with the dead . Some are simple enough: their bodies are collected, identified and put to rest with the minimum of fuss. They are the known deceased, with loved ones and mourners. Some, though, are trickier, and require exploration to make sense of.

Peter Bergmann's body had been taken to post mortem. He had been found naked, his clothes scattered across the shore. The pockets were empty. No money, no wallet, no forms of ID. It was quickly established that he'd drowned, though there wasn't any hint of foul play. His teeth were in good condition, excluding a few fillings. It was his body that drew attention. It was battered and wrecked. The tests revealed advanced prostate cancer and bone tumours. He had suffered previous heart attacks and was missing a kidney. The toxicology report returned no evidence of medication in his system, despite the intensity of the pain he must have been suffering.

One even suggests that the whole thing was an elaborate hoax, cooked up by Irish filmmaker Ciaran Cassidy – whose 2013 documentary The Last Days of Peter Bergmann was until recently some of the only media coverage of the case – as some kind of avant garde comment on our macabre fascination with true crime. I asked Ciaran about this directly on Twitter and he responded a couple of minutes later. "It's real," he messaged back. "Welcome to the rabbit hole."

The years have spawned all sorts of wild, mostly online theories. At the time of writing, I counted nine separate Reddit threads dedicated to the mystery of Peter Bergmann. Some posit he was an intelligence operative, or a gangster on the run from a shadowy organised crime group. Others, that he was trying to claim a life insurance policy for his loved ones.

Despite the dead ends and false starts, Ray explained how many hours have been dedicated to the hunt for answers. Almost everything in their power has been attempted. They have conducted searches and chased down leads, no matter how far-fetched. They have Bergmann's DNA, clothes and remains. It is now a waiting game may go on forever. "I liken it to a computer that has gone into 'sleep mode'," he said. "When something new comes up, or someone credible comes forward, then we will move the mouse and it will spring back into action."

Ray believes Sligo was no random choice of destination. "There seems to have been a purpose to it," he said. "Everything he did seemed to have had a purpose, from cutting the labels of his clothes and all the rest of it. The question you have to ask is: why Sligo? If you want a scenic place to die, you're spoilt for choice across the west coast of Ireland, or even Scotland for that matter. Something must have brought him here, even if we've never been able to say what that was."

It was early evening when we met at Sligo's Riverside Hotel, and – after much back and forth on Twitter – I could finally ask how she came across the case. Despite growing up in a small town only a few miles from Sligo, Treasa had never heard of Bergmann before she wrote the play; she'd stumbled across him after searching for "unidentified missing people, west coast Ireland", and had been unable to stop reading. The story seemed to stir something in her, empathy and creativity in equal measure.

With Peter Bergmann, there are no loved ones that we know of, and only professionally concerned advocates pressing for answers. Instead of deep memory and grief, we have snatched recollections gleaned from a cluster of chance encounters. The taxi driver who remembers his courteous, softly spoken passenger. The people at the beach, who couldn't have known they were witnessing the strange figure's final moments. There are those that believe the initial hunt was aborted too soon. That somewhere, someone must remember or hold the key to his real identity. But despite the ongoing interest – buoyed by a recent Irish Times podcast about the case – Ray Mulderrig told me that no one has ever come forward with anything truly convincing.

When someone is reported missing , we are used to seeing their grieving family and loved ones representing their concerns. Homemade posters are printed, campaigns are organised and coordinated. There will be someone there to carry the heartbreak long after police resources and media interest have run dry.

We talked over what we knew of the case. There were the cut up clothes and the ghostly room at the Sligo City Hotel. There was the grainy hotel CCTV footage showing the condemned man making his way to and from his final errands, the substance of which we can still only guess at. There is the documentary and play, as well as all the theories and speculations of the online sleuths, transforming the patchwork of concrete details into an intricate tapestry of conspiracy. Finally, there are those who were left behind, whoever they may be.

"It's too intrusive to say that 'I wrote from his perspective'. You can't put thoughts in this man's head," she said. "But creating the backstory for him painted a picture for me, at least. I hope he had a nice childhood and a good life, though we can't ever be sure. Everything he left us with is just so sad. To know that you've seen the play and it's sparked something in you makes me happy, because I want him to be identified. Of course people want to know the answer, but he didn't want to be identified. Maybe he thought no one would care, I don't know."

In Sligo, every unsolved case is a matter of the same priority, and there are always several investigations on the go at any one point, as Ray explained: "[With a ] long-term case like Peter, it's exactly the same [as any other]. There are four of five long-term cases at the moment, including one woman from 2011, which we are treating as a murder inquiry. We go, we search, we look. And in some cases we never find the person."

Peter Bergmann, the man who transformed himself into a ghost, is a threat to our expectations of what a missing persons case is supposed to look like. There is an unofficial spectrum that runs from an everyday vanishing, through to the cases that become myth, or are supposed to represent something broader than the sum of their own facts. People go missing all the time, for all manner of reasons. Of course, they may have been taken and come to harm. They may be lost to us forever, having chosen to leave their life behind.

"We have a standardised format that we follow for any missing persons investigation," he explained. "Sometimes people just go missing for a short period of time. There are people who take their own lives in circumstances that mean we never recover the body. We had a missing person from here in 2008 who we suspected might have been murdered. We identified them eight years later with assistance from the Welsh police, through fingerprint technology. Peter is unusual. We don't have a missing persons report and never have. No one has ever come forward to say that this could be my father, brother or cousin."

In 2015 it was reported that no one could really say how many unidentified bodies were buried in Ireland, or being housed in its morgues. Most missing persons cases are resolved within hours, or days, just as they are in the UK and around the world. The teenage runaway comes home, the vulnerable adult is located, things return to what they were before. But that is no excuse to neglect those who remain lodged out of sight. For every Peter Bergmann who grabs attention and headlines, there is a case like that of the male skull that was recovered at sea in February of 2006. Estimates placed him as between 25 and 45 when he died, and likely of North African descent. The skull had been in the water for less than a year. Interpol were contacted and a DNA profile circulated, though nothing has ever come back and the case remains shrouded in silence.

Every hour of every day sees someone reported missing in Ireland, at a rate of around 9,000 reports a year. According to figures compiled in 2015, the average length of time for a person to be officially listed as missing by gardaí is over ten years, while the oldest open case dates back to 1967.

Peter Bergmann's story takes all of this and flips it into something that feels both new and strange. We know that he chose it all, from his pseudonym to the place and time of his death. Perhaps his story was an extreme manifestation of taking back control. He was at the end of illness and wanted to die, so he did it, with a rare kind of premeditated thoroughness. There was a death sentence deep in his bones and heart, but the remainder of his time was his and his alone.

Before my second visit to Sligo I'd spoken with Tosh Lavery, an ex-Garda who'd spent 30 years in the Sub-Aqua unit, investigating some of Ireland's most infamous murders and missing persons cases. Since his retirement at the start of the decade, he has worked with the families of missing people across the country to highlight their plight and drum up interest when it wanes. Tosh is a vocal advocate for the missing and has his own thoughts on Peter Bergmann. Solving the case is a moral issue, just as all missing persons cases are to Tosh. He told me how he much he hates the word "closure", which crops up so often in any conversation involving the missing, Bergmann included.

"I don't know what it means when people say that," he told me over the phone. "Even if we find the person and get to the bottom of their story, it doesn’t mean that it makes up for all the ambiguity that people have had to live through."

The more I thought about the man who called himself Peter Bergmann, the more I started to doubt the motives for my own search. He had tried to cover his identity so thoroughly that it would never be discovered. The forensic attention to the circumstances of his own death spoke of a man who didn't want to be remembered, for whatever reason.

Did I, or anyone else, have the right to reject that statement of intent, in the name of curiosity. And what was it that I even hoped to find? Like Tosh, I questioned what closure could mean for Peter Bergmann. Does our urge to know outbid his right to be forgotten? There are many different answers, each with their own partial and unsatisfactory truth. But Peter Bergmann does not stand alone; his story made me think of another 21st century case that had gripped the frenzied attention of online sleuths and baffled law enforcement.

In September of 2001, a 25-year-old man checked into a motel in a village in rural Washington, using the pseudonym Lyle Stevik. His body was discovered several days later, with an immediate verdict of suicide. He had left a note and some petty cash, but had spent great pains obscuring his identity. As the years passed and leads grew cold, a dedicated community mushroomed around his memory, trying to crack the puzzle to a sad, sorry story.

In 2018, there was a breakthrough. DNA analysis led law enforcement to the man's family, who had lost touch many years before his death. They had thought him alive, and that he had simply cut ties with them and left for a life far away from his beginnings. The family appealed for privacy and the specifics have never been released, at their request.