In the Alpine foothills in the far south of Germany is a vast lake called the Ammersee. Its shores are dotted with centuries-old villages where wealthy families from Munich buy large second homes and tourists drink beer at waterfront restaurants. At the north end of the lake is a pair of such villages, Eching am Ammersee and Schondorf, less than two miles apart. Separating them is a block of spruce forest that attracts hunters, joggers, mountain bikers and in the late summer 38 years ago, kidnappers preparing to commit what would become one of the country’s most notorious postwar crimes.

After class on Tuesday 15 September 1981, the first day of the new school year, a 10-year-old girl named Ursula Herrmann returned to her house in Eching. Ursula, the youngest of four siblings, practised piano with her oldest brother Michael, and then headed off to her late afternoon gymnastics lesson in Schondorf, cycling through the forest along the lakeside path. When the gym class was over, she went to her cousin’s house in Schondorf, where she ate dinner. At 7.20pm, Ursula’s mother phoned the aunt to say her daughter needed to come home. The shadows were lengthening but it was still light, and the cycle ride would only take 10 minutes.

Half an hour later, she was still not home. Her mother again called the aunt, who said Ursula had left 25 minutes before. Both of them immediately knew something was wrong. Ursula’s father rushed into the forest from Eching, and her uncle did the same from Schondorf. They met in the middle, along the path. Ursula’s name rang out through the darkening wood. But there was no reply.

Within an hour neighbours, police and firemen had joined the search, torch beams raking the water and struggling to penetrate the thick undergrowth. With midnight approaching, and rain falling, a sniffer dog led its handler away from the lake, into the brush. There, 20 metres from the path, was Ursula’s little red bike. But she was nowhere to be seen.

Ursula Herrmann, aged 10.

Photograph: Lka Bayern/EPA

At first light the search intensified. Dozens of officers wearing raincoats and rubber boots spread out through the dense forest, on the border of which stands Landheim Schondorf, an expensive private school founded in 1905 and favoured by Bavaria’s political and business elite. As a helicopter hovered overhead, a police boat and divers scanned the shallows of the lake. Local radio carried the shocking news of the missing girl in an idyllic part of the country: 1.43m (4ft 7in) tall with short blonde hair, wearing dark green cords, a grey woollen cardigan and red-brown sandals; the daughter of a teacher and a housewife.

On the Thursday morning, when Ursula had been missing for more than 36 hours, the phone rang in the Herrmann house. When Ursula’s parents picked up there was silence, and then a short, familiar jingle, which they recognised from the traffic bulletin on the Bayern 3 radio station. More silence ensued, and then the jingle played again before the caller hung up. Three more similar calls – baffling and sinister – followed over a period of hours. A team from the local police department, now stationed inside the Herrmann home, began recording the calls.

At noon the next day, the postman delivered an envelope addressed to Ursula’s father, marked urgent. Inside was a ransom note composed using letters and words cut out from tabloid newspapers. “We kidnapped your daughter,” the note began, in broken German. “If you ever want to see your daughter alive again, then pay 2m deutschmarks [£450,000] ransom.” The kidnappers, expecting the letter to have arrived a day earlier – before the calls began – explained that they would phone the Herrmanns using a jingle as their call sign. “Just say if you will pay or not pay … if you call the police or do not pay we will kill your daughter.”

When the phone rang that afternoon, and the jingle sounded, Ursula’s mother agreed to pay the ransom. She also asked for proof of life: what were her daughter’s nicknames for her two stuffed toys? When the kidnappers did not reply, she became frantic. “Talk to me, say something, something from Ursula!”

That same evening, the kidnappers posted a second letter, which arrived on Monday 21 September, with curiously specific instructions regarding the ransom. The kidnappers wanted the money to be paid in used 100-deutschmark bills, packed in a suitcase. It was to be delivered to an as yet unnamed location by Ursula’s father, who was to drive alone in a yellow Fiat 600 going no faster than 90km/h.

One of letters sent by the kidnappers, composed of newspaper cuttings. Photograph: EPA

Unlike some other residents of Eching, and the parents of the pupils at the boarding school in Schondorf, the Herrmanns were not wealthy. They had only been able to build a home near the lake because Ursula’s great-grandfather had purchased some grazing land there decades earlier. A neighbour raised part of the ransom, and the state agreed to cover the rest.

The Herrmanns waited desperately for more instructions. But there were no more letters and no more calls. Nor did the police have any strong leads. Two weeks passed. The police decided to search the forest again. More than a hundred officers were assembled, with 10 sniffer dogs. The wood was divided into four parts, and each quarter into small grids. The teams began searching every grid, one by one, using metal rods to probe the ground.

By the fourth day of searching, a gloomy Sunday, they had covered most of the forest. Ursula had been missing for 19 days. At 9.30am, there was a loud shout. In a tiny glade about 800m away from the lake path, one of the officers had struck something solid when probing the soil. Another policeman rushed over and, after wiping away the leaves and scraping through a layer of clay, discovered a brown blanket covering a wooden board. He removed it only to find second board, which appeared to be the lid of a box. It was 72cm by 60cm – the size of a small coffee table – painted green and locked from the top with seven sliding bolts. Using a spade, he forced the lid open, and peered in. There was Ursula. Her body was cold, lifeless. The officer wept when lifting her out.

Two detectives were sent to break the news to Ursula’s parents at their home, a short walk away. While her mother was too distraught to ask any questions, her father asked repeatedly: had his daughter been hurt before her death? The truthful answer was no. An autopsy concluded that Ursula died within 30 minutes to five hours of being buried. Since there was no sign of struggle, or even movement, inside the box, the doctors assumed she had been drugged beforehand, possibly with nitrous oxide.

It appeared that the kidnappers had planned to keep Ursula alive. The box, 1.40m deep, was fitted with a shelf and a seat that doubled as a toilet. It was stocked with three bottles of water, 12 cans of Fanta, six large chocolate bars, four packets of biscuits and two packs of chewing gum. It also contained a small, bizarre library of 21 books, from Donald Duck comics to westerns, romance novels and thrillers with titles such as The Horror Lurks Everywhere. There was a light and a portable radio tuned to Bayern 3, the same station that broadcast the traffic jingle. To enable Ursula to breathe, the box had a ventilation system made from plastic plumbing pipes, which extended to ground level. But whoever designed it had failed to realise that without a machine to circulate the air, the oxygen would quickly run out.

The police believed they were hunting more than one kidnapper, because of the size and weight of the box. At 60kg, it would probably have needed at least two people to carry it into the woods. The perpetrators must have known the forest well, for they had chosen a remote site within it, and had avoided attention while digging the hole and hacking paths through the dense brush.

In Eching and nearby villages, parents who previously let their children roam free were now terrified to let them out of sight. The shock was amplified by the frenzied press coverage. On the day of the funeral, after much harassment from journalists, Ursula’s brother Michael, a shy 18-year-old, lost his temper with a photographer who held a camera right in front of his face, and knocked the camera to the ground.

Desperate to find the culprits, the police offered a DM30,000 reward for information, and tips poured in. One name that came up was Werner Mazurek. He was 31, lived with his wife and their two children just a few hundred metres from the Herrmanns. A trained car mechanic who left school at 15 and now ran his own TV repair business, Mazurek was good with his hands. He was imposing – tall, with a beer-drinker’s stomach – and quick-tempered, and not well liked in Eching. He was also heavily in debt, owing a bank more than DM140,000, so he had a motive.

A police sketch of the box in which Ursula’s body was found

Questioned by police a week after Ursula’s body was found, Mazurek could not initially recall his movements on the night she went missing. It took him 24 hours to provide an alibi: he had been playing the board game Risk with his wife and two friends. But a search of his home and workshop revealed nothing that linked him to the crime. Later that month, the forensics team examining the box found a fingerprint on a piece of duct tape, raising hopes of a breakthrough. Thousands of locals, including Mazurek, were fingerprinted, but no match was discovered.

The police still suspected that Mazurek was involved. At the end of January 1982, they arrested him, along with two of his friends, and interrogated them for several days before releasing them. A month later, another of Mazurek’s acquaintances was questioned. Klaus Pfaffinger was an unemployed mechanic with a drinking problem. His landlord, who was owed rent, had told police that in the weeks before the crime he had seen his tenant driving his moped with a spade strapped to the side. Pfaffinger initially protested his innocence. But on the second day of questioning, when the interrogators took a break and he was alone with the police secretary, he said a startling thing: “What if I know something?” When the interrogators returned, Pfaffinger told them that Mazurek had asked him to dig a hole in the forest in early September 1981, promising payment of DM1,000 and a colour television. Pfaffinger said he had dug the hole and had later seen a box embedded inside.

Convinced they had cracked the case, the detectives drove Pfaffinger to the forest that separated Eching and Schondorf. They asked him to lead them to the burial site. To their dismay, he was unable to locate it, or even come close. On returning to the police station, he announced: “I am revoking this confession, it’s not true what I said.” During at least 10 subsequent interrogations in the following months, he refused to repeat his confession, and was eventually released without charge.

By the summer of 1982, after his name was smeared, Mazurek was preparing to move away from Eching with his family. The lead detective who had pursued him was replaced, and the net cast more widely. Some 100,000 colour posters requesting help with the investigation were distributed nationwide. A television programme, Aktenzeichen XY ... Ungelöst – Case number XY … Unsolved – which would serve as the model for the BBC’s Crimewatch and America’s Most Wanted, featured a long segment on the Ursula Herrmann case. The new police team found more evidence of the kidnappers’ methods, including a wire that they had strung through the trees along the lakeside path to serve as an alert system during the abduction. But investigations of other suspects came to nothing. By the end of the 1980s, the investigation had wound down. Across Germany, most people still remembered the shocking unsolved case of the 10-year-old girl buried alive in the box.

Ursula’s parents and siblings, meanwhile, were doing their best to move on with their lives. Though they grieved deeply for Ursula, who they remembered as an intelligent, energetic girl who loved to sing and paint, they did so in private, never speaking to the press. Soon after their youngest child’s death, the parents had made a conscious decision not to let the hunt for the kidnappers consume the family, or the tragedy define their lives. Without any known perpetrators to blame, they tried to think of it as a terrible accident. It was hardest for Ursula’s mother, who believed she should have gone to fetch her daughter from her cousin’s house. Ursula’s father and sister turned to their strong Christian faith to find peace. Her youngest brother eventually found solace in surfing.

A model of the box on display at the district court in Augsburg. Photograph: Karl-Josef Hildebrand/EPA/Shutterstock

Michael, the elder brother, who was in his final year of school at the time of the crime, was playing music at a friend’s house on the night Ursula disappeared. When his mother called in a panic, saying his little sister was missing, he rushed home and joined the search for her in the forest. He was devastated when her body was found. “Then it quickly turned into: what can I do with this now?” he told me recently. “Because I knew the ‘why’ could never be answered. I decided: I am alive and I have some tasks to do.”

In the mid-2000s, the Bavarian state office for criminal investigations started looking in earnest at its backlog of cold cases. The most famous was the Ursula Herrmann kidnapping, which had by then appeared three times on Case number XY … Unsolved, and was still a stain on the reputation of the local police and judiciary. Prosecutors hoped that the development of DNA profiling over the previous two decades might help crack the case. The mass of evidence from the original investigation, including the ransom notes and the box, was painstakingly re-examined. Numerous hairs were found, from which the forensic experts were able to build the DNA profiles of several different people. Now they just needed a match. In 2007 they got one.

A genetic sample recovered from a screw on the box matched that found on a glass in the Munich penthouse of a wealthy woman who was brutally murdered there in May 2006. But the police excitement about a breakthrough was short-lived. On trial for the Munich killing was the victim’s nephew, who was only a few years old when Ursula was kidnapped. After extensive forensic investigations, the judges ruled that no link could be established between the two criminal cases, and the nephew was convicted for the Munich murder. How the match with the sample from the Herrmann case occurred remains a mystery – though very rare, mistakes do occur in genetic profiling.

For the prosecutors looking at the Herrmann case, time was running out. Her death had not been deemed a murder, but rather kidnapping with deadly consequences, a crime that carried a 30-year statute of limitations. In five years, the people responsible would be in the clear. The state prosecutors went back to the 1980s case files, to look at the main suspects. Klaus Pfaffinger, the unemployed man who briefly claimed to have dug the hole, was dead. But Werner Mazurek was still alive, and living with his wife in the north of Germany, where he ran a boat accessories businesses and, with a friend on Tuesday evenings, a snack bar that bore the advertising slogan: “Norbert’s pig and Werner’s beer, the finest at the harbour pier”.

In 2007, Mazurek was placed under surveillance and an undercover officer deployed to befriend him. Police planted recording devices in his car and his house, and tapped his phone. In October that year, his home was searched and he was asked to provide a saliva sample. It did not match any of the genetic profiles found on the box.

Ursula’s brother Michael at the court during the murder trial in 2009. Photograph: Dpa Picture Alliance Archive/Alamy

The prosecutors had one hope left. Among the items taken from Mazurek’s house during the search was an old reel-to-reel tape recorder. In the calls to Ursula’s parents in the days after her disappearance, the kidnappers had played a jingle. Was it possible that this device was used to record that jingle from the radio all those years ago? A sound expert, who had access to the original recordings of the 1981 calls, spent months conducting tests on the tape recorder, concluding that it was indeed used in the kidnapping.

On 28 May 2008, nearly 27 years after Ursula’s death, Mazurek was arrested and flown to Augsburg, a city near Eching. Ursula’s parents, who were still living in the same house by the Ammersee, had been notified a few days before that an arrest was imminent. They were also told they could be part of the trial. Under the German legal system, relatives of the victims of certain serious crimes are allowed to formally join the prosecution as nebenklage, or co-plaintiffs. This gives them the right to view the evidence, request witnesses and put questions to the judges.

Ursula’s parents did not want to be confronted again by the horrifying details of their daughter’s death, all these years later. Instead, it was agreed that the co-plaintiff would be their oldest son, Michael, who was by then in his 40s, teaching religion and music at a girls’ secondary school in Augsburg. He was a quiet, family man, but also one who is “not content with half-truths”, as his old friend Michael Hofstetter, who was with him in Eching on the evening Ursula disappeared, recently told the Süddeutsche Zeitung newspaper. “He has such a very deep sense of justice that drives him.”

The trial opened in February 2009 before a packed court in Augsburg. Mazurek, characterised in one newspaper as a “bearded giant”, sat in front of his wife, who was also on trial as an accessory to the crime. Reading from a 20-page statement, Mazurek insisted he was innocent. “I know I was certainly not a good citizen, sometimes rude, and we’ll see many attempts to portray me as a bad person. But I have nothing to do with this act.”

The prosecution had no difficulty finding evidence of his poor character. Mazurek’s daughter and stepson had few good things to say about him as a father. He had also had other scrapes with the law, including a fraud conviction in 2004 for falsifying documents. Then there was the story about the dog. In 1974, Mazurek returned from the Oktoberfest beer festival to find that the family dog, a mixed-breed named Susi, had overturned the rubbish bin in the kitchen. Mazurek grabbed the dog and locked it in the basement freezer. The next day his wife at the time, who would soon divorce him, went to the freezer to get some meat only to discover Susi there, frozen to death. Mazurek later said he had punished the pet “with exile to Siberia”.

The prosecution laid out the circumstantial evidence against Mazurek. He had a motive as he needed the money, and the means to secretly build a box, because he owned a workshop. While Ursula was missing he had been observed listening to police radio, and a piece of leather used in the box construction was cut from a belt owned by someone with a large stomach, like Mazurek. And, in 2007, after police searched and bugged his house, they listened in on a phone call between him and an old friend from Eching where they discussed the statute of limitations for the Ursula Herrmann case.

But the key elements of the prosecutors’ case were the revoked confession by Pfaffinger – that he dug the hole at Mazurek’s request – and the tape recorder. They insisted the confession was credible. As the old police files showed, Pfaffinger’s confession was accurate in several ways: he had described the burial site in detail, from the size of the forest glade and the dimensions of the hole to the soil conditions. The lead police investigator in 1982 was convinced Pfaffinger deliberately misled him during the forest visit, when he could not locate where the box was buried. Testifying in court, all these years later, the same policeman described Pfaffinger as an “excellent actor and practised swindler”.

The tape recorder was the most important, and controversial, piece of evidence. When questioned by police in 2007, Mazurek said he had purchased it only a few weeks earlier, at a flea market while on a short holiday with his wife. But he could not prove who sold it to him, and nobody at the market could recall such a device being on sale that day. The state’s expert, whose speciality was phonetics rather than audio, described how, in the recording of the ransom calls, you could hear a couple of clicking sounds – the buttons of a tape recorder being pressed during the recording of the jingle. When she pressed the buttons on the tape machine, she had, in her words, an “aha” moment. The sounds were identical. Other subtle characteristics of the recording also corresponded precisely to the specific machine in front of her. It was, she said, “probable” that the very same tape recorder found in Mazurek’s house was used in the ransom calls.

Detective chief superintendent Detlef Puchelt shows a picture of the tape recorder that was used as evidence. Photograph: Christof Stache/AP

Summing up, in March 2010, the senior prosecutor reminded the court that Ursula had been “buried alive in a box”, revealing the “cold-bloodedness and mercilessness of the perpetrator”. The three judges and two jurors were convinced, finding Mazurek guilty and sentencing him to life imprisonment. His wife was acquitted, due to lack of evidence. In court, everyone seemed happy that Ursula’s killer had finally been put away. Everyone, that is, except one person.

At the start of the trial, few people in court had taken much notice of Michael Herrmann. Despite his distinctive appearance – he wears his grey hair in a ponytail and at the time also sported thin sideburns down to his jaw – he is unassuming, not the kind of man who draws attention to himself. After graduating from high school, he studied teaching in Augsburg, and then opened a music store, selling instruments and equipment, including tape recorders. He got married, had three children and fostered a fourth. He loved watching them grow up, seeing them share his love of music, finding their own way in life.

People who knew what happened to Ursula sometimes asked if it made Michael anxious about his own kids, but for some reason it didn’t. Nor did he ever think about looking for the perpetrators himself; that was the job of the police. Though he felt at peace in his life, his sister’s death still felt to him like an “unclosed circle”. The trial, and his status as a co-plaintiff, offered an opportunity to close it. While most nebenklage are passive observers in court, Michael decided to take his role far more seriously. He would not allow the family to be victims a second time.

Before the trial started, to the surprise of his state-assigned lawyer, Michael had requested full access to the case files, which ran into tens of thousands of scanned pages. In the first few weeks of the trial he got through 6,000 pages, locking himself in the study at home at night, unable to stop reading. His memories of Ursula were strong: he recalled how, despite her liveliness, she was also cautious and at times sensitive, growing upset when some of her schoolmates repeatedly misbehaved. But reading through the typewritten police reports he realised he had forgotten many of the details of the horrifying days in September 1981, even the fact that he had helped Ursula with her piano practice just a few hours before she was kidnapped. It was, he felt, like his brain has somehow blanked out that part of his life. To Michael, there was much to suggest Mazurek might have committed the crime, but there were also things that troubled him about the prosecution case. He could not understand why Pfaffinger’s revoked confession was now being treated as plausible, when it was dismissed all those years before. From the police files it was clear that Pfaffinger had a serious alcohol problem. While in detention he claimed to have experienced hallucinations. He was also chronically work-shy; questioned in 2008, his former wife called him a “lazy guy” who would never have agreed to dig a large hole.

As the court heard, Pfaffinger’s confession was not even signed; the investigator wrote it down from memory weeks later. And, as with Mazurek, there was no DNA proof connecting Pfaffinger to the crime. Before the trial, the police exhumed Pfaffinger’s body but there was no match to the genetic profiles they had discovered among the evidence a few years before.

Most concerning to Michael was the tape recorder. With his background in music, he knew a lot about acoustics and sound engineering, and could not understand how a tape recorder could be definitively linked to the ransom calls all those years ago. Even if the reel-to-reel device had been used to record the jingle from the radio, as the prosecution alleged, the kidnappers would still have had to transfer that recording to a second, more portable device, since the calls to the Herrmann house were made from pay phones. The acoustic environments in the booth and at the kidnapper’s home would also have influenced what the police eventually heard and recorded at the other end of the phone line.

Michael’s lawyer advised him not to make a big deal out of it. “She said: ‘you don’t do this as a nebenklage’. But I did not think about habits, I just did what I thought was right,” Michael told me. He wrote a letter to the court, calling the sound expert’s report about the tape recorder “incomplete or one-sided”. The judges were not happy, but by law they were obliged to read out the letter in court. It was a highly unusual and sensational intervention – a member of the prosecution team, the victim’s brother, no less, acting in favour of the defence. When the verdict against Mazurek was announced, Michael made a statement at the courthouse. “I am not convinced of his guilt,” he said. “But neither am I convinced of his innocence.” Instead of the circle being closed, it had opened further.

Six months after the trial, in late 2010, Michael began to notice a strange, high frequency noise in his left ear. At night the hissing would wake him up and prevent him falling back asleep. Even worse, it often tormented him during the day, especially when was he trying to teach music. He had never experienced tinnitus before, and thought it might relate to the trial. The court-appointed psychologist, on hand to assist relatives of crime victims, examined him and agreed that the stress of the court case was indeed the likely cause.

During the trial, Mazurek had sent Michael a letter – not to thank him for questioning the tape recorder evidence but to suggest that they were somehow on the same side. From prison Mazurek kept writing, and even sent a Christmas card. In 2013, Michael finally replied. “I was surprised to receive a letter from you, because it is certainly clear to you that despite all the doubts I have about your guilt, I have considerable reservations about your person,” he wrote. “If you are not the culprit I wish [for] more new insights and that you can be rehabilitated. If you are the culprit, go to hell.”

By then, Michael was increasingly sceptical that Mazurek was guilty. After the trial ended, Michael kept returning, night after night, his ear ringing, to the case files, which he had stored on his computer, meticulously arranging the evidence in folders. It had put strain on his marriage – he separated from his wife in 2012 – but he could not let it go. He felt he owed it to his parents, to himself, even to the German public, to pursue the truth. “What drives me is ethics – doing what is morally right,” he told me. “It was just wrong for the case to end like it did.”

Werner Mazurek in court Photograph: Christof Stache/APN

So he came up with a plan. In 2013, he filed a civil claim, seeking €20,000 in damages from Mazurek for causing his tinnitus. It was a legal ruse: since Mazurek would defend the case on the basis that he was wrongfully convicted and so could not be considered responsible, the court would have to reconsider the facts of the criminal trial before coming to a conclusion. It would be an opportunity, Michael believed, “to get nearer to the truth”.

“The judges knew what was going on, and they were pissed off,” Joachim Feller, who has been Michael’s lawyer since 2012, told me when we met at his offices in Landsberg am Lech, an ancient town about 20 minutes drive from Eching. “They tried several times to stop it going forward.” The court insisted that an independent psychiatrist examine Michael, and to rule on whether his tinnitus was caused by the trial. After the psychiatrist confirmed it was, the case finally went ahead in 2016. It dragged on for more than two years.

Unlike with the criminal trial, where the media focus was on Mazurek, it was now on Michael. He found himself having to explain to his pupils in his music and religion classes, who knew him as a reserved, kind teacher, why they were seeing his face in the newspapers and on TV. He took journalists to Eching, and into the forest where Ursula was kidnapped. Even so, apart from Herrmann’s close family and friends, few understood why he was pursuing the case. A local journalist who covered the criminal and civil cases told me his newsroom colleagues often asked him why Herrmann “could not just let it go”. “I myself am still trying to work out why Michael Herrmann is acting like this,” the journalist said. “He is quiet and calm, but still he looks into the files … There is a little obsession.”

As the civil trial went on, it became clear that he was not the only person with doubts about the original verdict. Appearing for the defence was a retired physicist and amateur sound expert named Bernd Haider, who had built his first tape recorder from scratch in the 1960s and lived in a village just a few miles from Eching. He vividly remembered the coverage of the crime from 1981, though he had never heard of Mazurek before his arrest. Haider had followed the 2009 trial in the media and, like Michael, was highly sceptical about the tape-recorder evidence. Later, he had borrowed a similar machine, got hold of the ransom call recordings, and tried to see if it was possible to replicate the phonetic expert’s findings. After a year of testing, he concluded it was not, and offered his assistance to Mazurek’s lawyer.

When I visited Haider this spring, the borrowed tape recorder was still in his loft. After a lunch of wiener schnitzel and potatoes, he told me: “Michael Herrmann was the only person in the original trial who understood what the problem with this evidence was. He said ‘it’s impossible’, but he was sitting on the wrong side of the court!”

Towards the end of the civil case, Michael gained another ally. In London, a German academic named Barbara Zipser read an article online about his efforts to get to the truth. Zipser was a child in Germany when Ursula was kidnapped, and recalled the horror she felt then. In terms of its impact, it was the German equivalent of the Madeleine McCann case, Zipser told me when we met this year. “I thought: whoever did this, I want that person in jail,” she said.

Since Zipser’s speciality is linguistic profiling – at Royal Holloway, University of London, she uses modern profiling techniques to identify authors of ancient Greek medical texts – she decided to compare the ransom notes sent by the kidnappers to samples of Mazurek’s writing, which Haider had posted on the internet. Zipser analysed the words used, and the writing style. Whoever composed the ransom notes was well educated, she said, a native speaker pretending to be a foreigner by writing in broken German. “I am sure it was not Mazurek,” Zipser told me.

Her opinion only hardened after she went to meet Michael in Germany, and spent many hours going through the case files with him. “I know this is an incredible story but I’ve seen the evidence, and Michael has done very good investigative work,” she said. “I support him in his findings.” For a few years after the criminal trial Michael thought there was still a 50% chance that Mazurek was the kidnapper. Now he puts it at 1%.

In August 2018, the civil case concluded and the court ordered Mazurek to pay Michael €7,000 for causing his tinnitus. It was a victory that to Michael represented a loss, since to arrive at the decision the judges first needed to agree with the criminal court that Mazurek, together with an unidentified accomplice, was indeed the man who had kidnapped Ursula.

In an open letter to the Bavarian state and the media, Michael wrote: “My sister’s fate has stayed with me for 37 years, and to this day, it is unclear who was actually responsible for her death. Could it be that the Augsburg legal system is not actually interested in solving the case of Ursula Herrmann, the death of my little sister? … If the court decides to close the proverbial lid, it should be well aware that one cannot shut the truth away.”

Since Mazurek was arrested in 2008, he has been represented by Walter Rubach, one of Bavaria’s best-known defence lawyers. Rubach sees things in black and white. If a client asks him whether he believes in their innocence – as Mazurek did in 2008 – he brushes away the question. “I told him I don’t believe any of my clients,” Rubach told me in his chambers in Augsburg. “My job is to work out if there’s enough proof and evidence to convict them.” In Mazurek’s case he was convinced from the start that there wasn’t.

“It was clear that Mazurek was a person who could have committed something like that. But there were no hard facts – it was a circumstantial case at its finest,” Rubach said. “I am still upset at how he was convicted. What is it you say in England? Rather let 10 guilty men go free than hang one innocent one.”

Though Rubach has had little personal interaction with Michael, he has watched him across the court, and likes and admires him. “What he has done, as a co-plaintiff, going against the decision of a court – this never happened before in Germany.”

From his prison cell in Germany’s far north, Mazurek is still trying to clear his name. After I wrote to him this year, he replied saying he had hired a private investigator to track down the man who he says sold him the tape recorder in 2007. “I am just angry and I am awaiting the 11th anniversary of my time in jail,” he wrote.

On a mild Sunday morning in April, I met Michael in Augsburg. As he had been most days in court, he was dressed casually, in trainers, blue jeans and a black jacket. Despite his legal setbacks, and the tinnitus that continues to bother him, he comes across as calm and warm, and possesses a dry sense of humour. As we drove through the Bavarian countryside towards Eching, he tried to explain the meaning of überfordert, the word he used to describe the police in 1981: “It means when the task you have is bigger than your capabilities – like Brexit.”

Michael knows the case material so well – he has put in far more hours of research than any of the lawyers for the defence or prosecution – that when he speaks, it is with the precision and detachment of a special investigator, rather than a relative. After parking the car beside the road between Eching and Schondorf, he noted it was probably where the kidnappers had parked when bringing the box to the forest. An SUV track led into the woods. “We need to walk 141 metres,” Michael said, and then look 20 metres to the left, to locate the spot where Ursula was buried. “We don’t know if she was sedated and carried or if she was forced to walk there,” he said. “But we know she was taken on paths specially cut through the forest.”

Michael’s father died several years ago, and in 2016 his mother moved out of the family home in the village to Augsburg. But his younger brother Hannes – the surfer – still lives in the house, along with two Syrian refugees who rent out the bottom floor. Michael phoned him – he did not want to show up with a journalist, unannounced – and Hannes invited us in for coffee. We never spoke about Ursula. As with his oldest sister and their mother, Hannes has never talked to the media about his sister’s death, though Michael says that in private his family supports his work on the case, as do his children and his new partner. But in public, at least, he is alone in his quest to reopen the case.

After lunch at a restaurant beside the lake, where sailboats bobbed in the distance, we walked along the road towards the forest, the route Ursula took on her way to gym class nearly 40 years ago. The spruce trees are much taller than they were then, and the undergrowth less thick, but the path is the same: three metres wide, compact gravel. Cyclists on electric bikes sped by.

After a few minutes, as we neared a small jetty leading to a wooden hut used by bathers, Michael stopped on the path. “This is where Ursula was kidnapped,” he said. “It’s where her bike was found and where the bell-wire ended.” The bell-wire is the 140m-long coil of insulated copper wire the kidnappers used as part of a warning system. Though the police had noticed the wire while searching for Ursula, they only learned of its significance more than a year later, when investigators visited the private boarding school in Schondorf to talk to pupils about the case. Two students came forward, saying that seven or eight months after the kidnapping, when chasing an owl through the forest, they had found the bell-wire strung through the trees next to the lakeside path. The boys then did a very strange thing: they took the wire down, measured it on the school athletics track, and then kept it in their dormitory in a locked box, until the police visit.

On examining the wire, the investigators realised it must have been used during Ursula’s abduction. While one of the kidnappers waited for the victim, the other presumably served as a lookout further along the path, with their finger on a button that would light up a bulb or sound a buzzer at the other end of the wire.

Michael believes the wire is one of the key pieces of evidence that could help identify the real kidnappers. Besides hunters, joggers and cyclists, the boarding school pupils also knew the forest well. Yet it appears that none of them were fingerprinted at the time of the investigation. Another piece of evidence also hints at the possible involvement of younger people in the plot: an impression on the paper of one of the ransom notes revealed a mathematical probability tree, of the sort taught to teenagers. Michael also notes that in a comic found in the box, one of the main characters drives a Fiat 600, the car that was mentioned in the ransom note and which was rare in Germany at the time, suggesting the kidnappers may have read the comic.

Late last year, in an effort to have the case reopened, Michael submitted a dossier of all his new evidence and theories to the state prosecutor’s office in Augsburg. In April, when I interviewed Matthias Nickolai, spokesman for the public prosecutor, he acknowledged that many people still had questions about the verdict in the criminal trial, but insisted the judges had arrived at the correct decision in 2010, and that it was “final and absolute”. In August, this became official, when the prosecutor’s office announced it would not be reopening the case.

Through his lawyer, Michael told the local press that he would be making no more public statements about the case, nor giving interviews, which the media interpreted as him finally giving up. But when I emailed him recently he said this was not true. “I didn’t say that I am not going to take it any further,” he wrote.

On 15 September, a few days after I received that message, it was the 38th anniversary of Ursula’s kidnapping and death. As they have done for the last few years, Michael, along with his two siblings and his mother, travelled to the graveyard in Eching where Ursula is buried. Just the four of them, no partners. There they remembered the little girl who left her cousin’s house on her red bike on a summer’s evening and never came home. •