The story starts as dusk fell on Hagley Woods in Worcestershire on 18 April 1943.

Four young boys with a passion for bird nesting stumbled upon a heavily coppiced elm tree. The slightest of the group climbed up to peer inside its hollow trunk, where he discovered a human skull.

Terrified, they swore a pact of silence and fled – but Tommy Willetts, the youngest, was so traumatised that he told his father. Warwickshire Police recovered skeletal remains from the bole of the tree, along with remnants of clothing and finger bones dispersed around the trunk.

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Pathologist Professor James Webster concluded that the remains belonged to a woman aged 35-40, who had been placed “while still warm” into the tree where she had remained hidden for at least 18 months. Cause of death was attributed to asphyxiation, on account of a portion of taffeta found deep inside her mouth.

Police contacted every dentist in the country, hoping to identify the victim by her distinctive teeth. They also painstakingly eliminated all missing persons from the area.

After six months, with police no closer to identifying the victim or her killer, the appearance of graffiti across the region, asking “Who put Bella down the wych elm?” suggested that someone knew more than they were letting on.

Police honed their search to identify the graffiti artist and followed the trail of anyone from the area known as “Bella”. Neither line of enquiry was successful. The search of national dental records also proved fruitless; the woman in the wych elm had apparently come from nowhere and was missed by no one.

Two years passed. The case attracted the attention of the anthropologist Professor Margaret Murray, who clouded the investigation by citing a disturbing occult ceremony known as the “Hand of Glory”, theorising that the scattered hand bones indicated a ritualistic murder.

The press feasted on this latest detail, particularly when the body of local man Charles Walton was found in the nearby village of Lower Quinton, pinned to the ground with a pitchfork. Murray connected both cases and Scotland Yard appeared to take the theory seriously, to the further delight of the press.

By the early 1950s talk of witchcraft had taken hold of the popular imagination. Then in 1953, a woman calling herself “Anna” contacted the Wolverhampton Express and Star claiming to have known Bella’s killers. She met police in secret but details of her story were drip-fed to the public by a local columnist writing as “Quaestor”.

Anna sent the case in a new direction: espionage. She claimed Bella had been murdered by a German spy ring involving a British officer, a Dutchman and a music hall artist. It was highly plausible: the region’s many munitions factories had made it a prime target for Nazi intelligence-gathering designed to choreograph the Birmingham blitz.

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The public embraced the link between the Hagley Woods murder and espionage with relish; after all, this was the Cold War and James Bond had already made his debut in Ian Fleming’s Casino Royale. Police soon dismissed talk of the occult and concluded that the finger bones had been scattered by animals, not by a satanic coven. Despite Anna’s leads, the investigation began to gather dust.

In 1968, the writer Donald McCormick revisited both murders in his book Murder by Witchcraft. He asserted that Bella had been a Nazi spy (and occultist) named Clarabella, a woman well known to several senior Nazis, recruited by the Abwehr and given the code-name “Clara”.

He claimed to have gained access to Abwehr records which indicated that she had parachuted into the West Midlands in 1941 but subsequently failed to make radio contact and disappeared.

Furthermore, at least one piece of the contemporary graffiti, he claimed, referred not to “Bella” but to “Clarabella”. It was an entertaining, if boldly uncorroborated theory and did nothing to help bring the case to resolution.

Thirty years later, the mystery endured despite continuing media interest – including from The Independent, which revisited the story in 1999. But the official closure of West Mercia Police’s investigation and publication of the case file has allowed it to be re-examined – and a startling conclusion presents itself.

The police’s final review acknowledges that while there would be some merit in a DNA investigation, they have been unable to ascertain where Bella was laid to rest.

But they overlooked the fact that after the post-mortem, Bella’s remains were not buried by the local constabulary, but were in fact passed to one of Professor Webster’s colleagues at the University of Birmingham for more unofficial tests. The police were looking for Bella in the wrong cemetery.

McCormick’s theory may lack hard evidence, but in the 1960s there were only limited lines of enquiry available. He certainly couldn’t have had access to wartime MI5 files, which detail the interrogation of a Czech-born Gestapo agent named Josef Jakobs, arrested by the Home Guard after parachuting into Cambridgeshire in January 1941.

His declassified file at the National Archives contains a photograph carried by Jakobs at the time of his arrest, which throws McCormick’s claims into fascinating relief.

The woman in the photograph was named by Jakobs as cabaret singer and German movie actress, Clara Bauerle.

Jakobs told his interrogators that Clara was his lover and that they had first met in Hamburg where she had been singing at the Café Dreyer with the Ette Orchestra. She was well connected with senior Nazis and had been recruited as a secret agent. She was due to parachute into the Midlands after Jakobs had established radio contact, but he claimed that since he had been captured before he could send word, this was now unlikely to happen.

MI5 learnt that Bauerle had been born in Stuttgart in 1906, making her 35. She was indeed a cabaret artist – in fact, she spent two years working the music halls of the West Midlands before the war and was said to speak English with a Birmingham accent.

It isn’t difficult to see how the name Clara Bauerle might have been more easily remembered as “Clarabella” by English music hall audiences. And “Anna” would later allege a connection between “Bella”, espionage and music hall, in her letter to Quaestor in 1953.

The timings of these disparate strands of the story are remarkably convergent. Jakobs said that Clara had been due to parachute into the Midlands in the spring of 1941.

Curiously, there appear to be no gramophone recordings, live performances or movie appearances bearing her name after this date. Her singing career appears to have come to an abrupt end. McCormick’s Agent “Clara” parachuted into the West Midlands in early 1941 and subsequently failed to make radio contact.

Jakobs failed to convince MI5 that he could be reliably “turned”. In any case, MI5 noted in a memo that news of his capture was no secret, “on account of the inability of the Home Guard to keep their mouths shut”. On 15 August 1941, he was executed by firing squad, the last man to be put to death at the Tower of London.

Unless the mortal remains of the woman found in Hagley Woods can be located, the fate of Jakobs’ lover, Clara Bauerle, may prove to be as enduring a mystery as the question of who put “Bella” in the wych elm.