Poster boards of Jo Cox at a memorial event in Trafalgar Square (Photo by Matt Cardy/Getty Images)

Thursday is market day in Birstall. But on June 16, five years ago, the West Yorkshire village was even busier than usual. The Euros were in full swing, and by midday the local pubs had started to fill for that afternoon’s match between England and Wales.

Carrying a holdall and a plastic Tesco bag, Thomas Mair hovered on the market square with a baseball cap pulled low over his face. Jo Cox, the Labour MP for Batley and Spen, wasn’t due for her constituency surgery at the local library for another half an hour. As he waited, Mair stood and ate a Flake.

Five months later, he watched as CCTV footage of that day was replayed in Court 8 of the Old Bailey, occasionally taking notes in block capitals. From the press seats, I watched as Mair looked straight ahead, emotionless, as the devastating injuries he’d inflicted on Jo Cox were described: multiple stab wounds, two gunshots to the head and one to her body. As Mair attacked the 41-year-old MP, witnesses variously heard him shout “This is for Britain” or “Make Britain Independent”.

Within hours of Jo Cox’s death, commentators were blaming the toxic, highly-charged atmosphere fostered by the EU referendum. On the Leave side, any suggestion of shared sympathies with a racist killer, however remote, was greeted with contempt and indignation, and the perpetrator was cast as a mentally-ill loner.

Against this backdrop, the eight-day trial pieced together a forensic narrative of the murder, right down to the chocolate wrapper discarded in a bin in the market square. Mair’s guilt and motivation was never in question. In his sentencing remarks, the judge said: “There is no doubt that this murder was done for the purpose of advancing a political, racial and ideological cause namely that of violent white supremacism and exclusive nationalism most associated with Nazism and its modern forms.”

Mair had remained silent throughout the trial, and the judge refused his request to address the court after the guilty verdict. Prior to that, he had spoken just once during the entire criminal justice process, at the magistrates’ hearing a few days after the murder. Asked his name, he replied: “Death to traitors, freedom for Britain”.

What I found particularly unsettling was how, until the killing, this 53-year-old man had lived an anonymous life, with no known history of violence and no criminal record. That his first offence was the murder of an MP seemed extraordinary.

And so, soon after the trial, I wrote to Mair at Frankland prison, explaining that I was a crime writer interested to know what he would have told the court had the judge allowed his request. To my surprise, I received a short note in response. In block capitals, Mair told me he had written a three-page reply but that it had been confiscated by the prison’s censors. He did say he was open to a “dialogue”, but as far as I know my response to that never reached him. Instead, the prison informed me that any further communication was banned, and that Mair’s pages of thoughts had been “placed in stored property not to be issued until release”, even though his whole life sentence makes that an impossibility.

That dead-end left me to piece together the clues to Mair’s mind from the literature — presented by the prosecution — that he kept in his meticulously tidy bedroom and the scant traces he’d left on the far-Right scene.

In the late 1980s, Mair, then an unemployed man in his mid-20s living with his grandmother, was a supporter of the National Front, a party whose main policy was the repatriation of all non-white immigrants. The reclusive Mair does not appear to have been an active member. He also subscribed to the pro-apartheid magazine, S-A Patriot, sold through NF publications, sending in a letter in 1988 to complain that the British media only ever showed white South Africans in the “worst possible light”. A decade after the end of apartheid, Mair was still raging at the demise of the racist regime, blaming “white liberals and traitors”.

Delving deeper into the contents of his personal library, detailed at the trial, the idea that such an extreme individual could have been pushed to breaking point by pre-Brexit tabloid scare stories about migrants seemed to me implausible. Mair’s hero was not Nigel Farage: the contents of his bookshelves suggests it was Reinhard Heydrich, Hitler’s cruellest henchman. Holocaust denial and racial separation were his obsessions.  

By the late-1990s Mair was immersed in the ideology of the US neo-Nazi National Alliance which, according to a pamphlet he owned, dreamed of a “New White World” which would come into being after “a final cleansing”. A sort of bible for those on this most violent fringe of the far-Right was William Pierce’s novel The Turner Diaries, which depicted a future race war where feminised white men would “regain their manhood” and take revenge on “race traitors”. A passage from the novel was extracted in one of the books on Mair’s shelves, James Ridgeway’s Blood in the Face, imagining “thousands of hanging female corpses… They are the White women who were married to or living with Blacks, with Jews, or with other non-white males.”

That Mair was drawn to such views was particularly chilling in the light of his own family history. Born in Kilmarnock in 1963, Mair moved to West Yorkshire with his mother after his parents separated when he was 7. There she met Reginald St Louis, a Grenadian who had worked in the woollen mills of Huddersfield since arriving in the UK. The couple lived in Batley, while Mair was raised by his grandparents a couple of miles away in Birstall.

“Sometimes you look for the ‘wound’ and there is none, but in Mair’s case there was,” University of Kent criminologist and expert in terrorism Dr Simon Cottee tells me. Cottee believes that Mair could have felt abandoned, betrayed and resentful at his mother’s second marriage.

During the trial, Mair screwed up his face when the judge mentioned his internet search on “matricide”, made as he was researching Jo Cox, a mother of two. He reacted with the same contempt when her husband Brendan described the couple’s holidays in the Balkans volunteering with children orphaned by the war. As Mair was otherwise emotionless throughout, this response to mentions of mothers and motherhood stood out.

I was at the press conference after the trial when the senior investigating officer hinted that Mair, who had reloaded his gun after the attack on Cox, may have been planning to go on and harm his mother. Cottee, though, suggests the MP’s murder could have been a case of “displaced rage”: Jo Cox’s multicultural liberalism made her a “stand-in” or an “effigy” for the mother who betrayed Mair, but who he was unable to harm.

Mair had certainly harboured violent fantasies for many years. In his letter to S-A Patriot, he expressed a belief that “the White Race will prevail” though only after a “very long and very bloody struggle”. A decade later, in the aftermath of the London nail bombings, he ordered material from the USA on how to construct bombs and weapons. But he did not act on these thoughts until the summer of 2016.

It was only after the trial that the police revealed that the sawn-off .22 rifle Mair used to kill Jo Cox had come into his hands at some point between the summer of 2015, when it was stolen, and early June 2016, when Mair began to make Google searches on the MP at Batley library, typing sinister questions on how best to kill. If true, it would suggest that Jo Cox, MP for just over a year, was not a long-standing obsession, but had only drifted into his line of sight during the referendum campaign. The gun, it seems, came first. (The police still don’t know how he acquired the weapon, though they are sure he was not the person who stole it.)

Cottee described the rifle as a “rudimentary weapon”, not difficult to acquire with the right contacts. But the police characterised Mair as an “anti-social loner” of the most extreme kind; a man who sent just two or three text messages in as many years. He was not a pub-goer, had no criminal record and, as far as the police have been able to establish, no active membership or connection to any existing far-right organisations.

In the years since the trial, my requests for more information on this key issue have been met with terse responses from West Yorkshire Police. Recently, they announced that the active investigation into the gun’s origins had been closed, all avenues having been exhausted.

Cottee, however, is more intrigued by Mair’s use of a dagger, arguing that it revealed a desire to be close and intimate “in the killing moment”. Mair’s violence, fifteen controlled and precise stabs, was “too personal” and went “way beyond what was necessary”, he suggests. In what was Britain’s deadliest far-Right act of terror prior to Mair’s attack, David Copeland dreamed of sparking a race war with the nail bombs he set off around London. But Cottee thinks that for Mair, killing Jo Cox was an end in itself. When he was arrested in a cul-de-sac a mile from the crime scene, he was calm and passive.

“It was a moment of agency,” Dr Gwen Adshead tells me. The former forensic psychiatrist at Broadmoor, who I first met in the weeks after the trial, sees Mair’s crime as his attempt to make a mark in a world where he was “constantly on the receiving end of things”. She also sees a crime rooted in deep envy.

“Jo Cox had vitality and popularity and integrity and a partner and a family,” Adshead points out. The MP was the local success story: the Heckmondwike grammar schoolgirl who had gone on to Cambridge, lived an international life and later returned to serve the constituency where she’d been raised. “I am Batley and Spen born and bred, and I could not be prouder of that,” she’d said in her maiden speech.

By contrast, Mair, lonely and disconnected from society, had lived an empty life in his childhood home with his grandmother’s floral decor and his Nazi literature. “It may have made him feel that he was not alone,” Adshead suggested, when I asked about Mair’s fixation on figures such as Heydrich. “He thought he belonged to this group of people who felt the same way; that it was alright to have horrible, hateful feelings.”

Following the trial, the police disclosed that Mair had received treatment for OCD and mild agoraphobia. Mair’s mother, in her few words to the media, said that he had been treated for depression some twenty years earlier, which would have been around the time of his grandmother’s death.

Yet his life had contained shards of hope. He volunteered as a gardener at the grounds of Oakwell Hall, close to Fieldhead, an experience he described in 2010 to a local reporter as having done him more good “than all the psychotherapy and medication in the world”. I also met former colleagues of Mair at a scheme known as the Electronic Village which taught IT skills to disabled people. They talked of how Mair had arrived there to learn low in confidence and self-esteem, progressing to become a valued voluntary assistant.

But a few hours a week of human interaction and empathy could not unmake the long, slow shaping of a fascist life.

Adshead believes the “political fever” surrounding Brexit could well have provoked Mair’s rage. “But it is only one [factor] and often the final factors are very much to do with the individual perpetrator,” she clarifies, adding that Mair’s hatred had been trying to find a political cause.

“I would definitely think of the political environment as being a kind of “bicycle lock factor”, she explains, referring to a theory that a violent criminal needs multiple factors to align like the cogs in a combination lock in order to act. The final “number” that causes the lock to “spring open” can be something as seemingly insignificant as a look or a smile or a familiar phrase from the victim.

Adshead, whose new book describes her experiences of working with Britain’s most dangerous offenders, says the only chance of finding this final, usually deeply personal, “link” is by speaking to Mair himself, over many hours. Even then the answers may prove elusive.

But the only glimmer of access had been shut down. When I last tried to contact Mair in prison, I was told that any correspondence could be a “hindrance or distraction” to his “rehabilitation process”. It was an odd explanation for someone deemed beyond redemption by the court.

“We’re trying to put something rational around something fundamentally irrational and we can wear ourselves out trying to think about that,” Gwen Adshead said when I told her of my frustrations. “If we can understand why he did it then maybe we could control it next time. But ultimately it may make no sense.”