‘I WROTE YOU A THREE-PAGE LETTER CONTAINING SOME OF MY THOUGHTS,” the man who murdered Jo Cox told me, replying to a letter I had sent to him in prison. His note was short, just half a side of A4 paper, and there was no sign of the other three pages he mentioned.

Thomas Mair shot and stabbed the MP as she made her way to a constituency surgery at the local library in Birstall, West Yorkshire, on 16 June 2016, a week before the EU referendum. He told the two police officers who arrested him that he was a “political activist”. A search of the 52-year-old’s house revealed far-right and neo-Nazi interests. But once in custody, he gave police nothing more. “We’re asking you why you’ve done it,” an officer asked. “Jo Cox’s family want to know why she’s dead.” Mair sat, arms folded, defiant. When asked to give his name at the magistrates’ hearing two days after the murder, Mair replied “death to traitors, freedom for Britain”. But at the Old Bailey trial five months later, he resumed his silence, leaving a plea of “not guilty” to be entered on his behalf.

Jo Cox was the first female MP to be murdered in Britain, the first MP killed since Ian Gow was murdered by the IRA in 1990, but all we knew of Mair’s motivation were the slogans he shouted as he carried out his attack. Witnesses variously heard: “This is for Britain” or “Britain first” and even “Make Britain independent”. So after he was found guilty on 23 November, there was a stir in the press seats when the defence counsel requested that Mair be allowed to make a statement prior to sentencing. But Mr Justice Wilkie, presiding, was in no mood to risk a political outburst and further distress the victim’s family. Mair was taken from the court in silence.

While the ideology that drove Mair seemed in no doubt, I left the Old Bailey that day with many questions. We did not really know why he killed this popular MP – a 41-year-old with two young children. In some respects, the solitary, middle-aged man appeared an unlikely terrorist. After the verdict, Det Supt Nick Wallen described an “antisocial loner in the truest sense of the word”, someone so adrift that he could not even establish links with fellow fascists. Although he owned a mobile phone, Mair had sent just three text messages in as many years. The police had no idea how he had got hold of a firearm. Wallen also revealed that Mair had received treatment for obsessive compulsive disorder (OCD) and mild agoraphobia, but his pre-trial assessment had ruled out any severe mental illness.

I wrote to Mair directly a few weeks after the trial, when he was in Frankland prison. I had little expectation of a reply. Some weeks later, a letter arrived from the prison where Mair said he was being held “IN SEGREGATION ‘FOR MY OWN PROTECTION’”. He wrote in black biro, using capitals throughout. The brief note appeared to have cost some effort, every inky comma carefully considered. But the further three pages of thoughts Mair said he had composed for me had been blocked by the censor on what he complained were “SPURIOUS GROUNDS”. “I AM AT A LOSS TO SEE HOW WE CAN HAVE A ‘DIALOGUE’,” Mair wrote. “IF YOU STILL WANT TO TRY, THEN THAT IS FINE BY ME. BUT I DON’T KNOW IF EVEN THIS SHORT NOTE WILL MAKE IT PASS [sic] THE CENSOR.” A notice from HMP Frankland, attached, informed me that Mair’s letter had been stopped for “the protection of the reputation of others”.

Over the following months I struggled to assemble a picture of Mair’s life from other sources. Jo Cox was a passionate supporter of the EU and advocate for the rights of refugees, particularly women and children fleeing conflict in Syria, and there were calls from figures including Barack Obama to unite to fight the hatred that had killed her. But within little over a year, terror attacks devastated Nice, Istanbul, Manchester and Barcelona, white supremacists were celebrating the election of Donald Trump and anti-immigrant nationalist parties were gathering strength in Europe. In the UK, hate crime was on the rise. Those shouting “Britain first” were in the ascendancy. The open, liberal country that Jo Cox represented was in torment.

The police had highlighted Brexit as “a motivating factor” for Mair. The murder of Jo Cox soon became part of the post-vote battleground. Remainers argued that Mair was at the extreme end of the leave spectrum; Brexiteers rushed to cast him as a deranged loner. Any hope of society learning something from the tragedy was lost. Streets and squares were named after Jo Cox in Belgium and France, while in our own bitterly divided nation MPs increasingly faced verbal and physical abuse. The language of far-right demonstrations – threats against “traitors” and “enemies of the people” – began to pervade our parliamentary democracy. Boris Johnson has dismissed fears about inflammatory talk as “humbug”, but the example of Jo Cox’s killing has been used in a number of threatening letters sent to pro-remain – especially female – politicians. Three years on, the atmosphere is still full of aggression and mutual recrimination.

I first arrived at Batley station a week after Jo Cox’s murder and just hours after the shock EU referendum result. The constituency of Batley and Spen was one of many northern, former heavily industrial areas to choose leave against the urgings of local Labour MPs. “We’re pretty much all Brexit here,” a large man in a red Mickey Mouse fleece told me as we made our way towards the town centre, past semi-derelict mansion houses. One, with elaborate stone carvings, had tatty signs taped to the windows advertising cheap shots of alcohol. This former industrial powerhouse, the centre of the heavy woollen industry, had seen better days. The leave vote here was 60%, close to the depressed former coal mining towns to its south where two-thirds of voters had rejected the EU.

Facebook Twitter Pinterest Jo Cox delivering her maiden speech to the House of Commons in June 2015. Photograph: Parliament TV

Jo Cox had been one of their own here: raised in Heckmondwike and Batley, she was educated at the same grammar school where her parents had met. “She seemed like a nice lass but politically she weren’t my cup of tea,” the man in the fleece replied, in the weary tone I would hear a lot. He said the murder had made no difference to the anti-EU vote. “We’d already made up us minds,” the man told me, as we came to a halt outside an old Methodist chapel that now houses the Europa Bed company.

At the top end of Batley’s high street – with its Greggs, Betfred, a few charity shops and the back end of a large Tesco – are reminders of a more prosperous age, the Victorian town hall and opposite it the grand stone building of Batley library with its central clock tower. Austerity had threatened the future of this and other libraries in the area, which worried the new MP. Writing in a local paper in August 2015, Cox called them a “lifeline for job hunters without their own computer, slimmers, walkers, discussers, knitters and natterers”.

Mair was a regular library user, though he rarely borrowed books: with no computer at home, he came to access the internet. On 7 June 2016, Mair consulted the Wikipedia page of his local MP and watched a YouTube video of an American man shooting a .22 rifle in a field. It was nine days before the murder.

Investigators believed that Mair had got his hands on a weapon shortly before his attack, though they could not be sure. A gun like the one in the YouTube video had been stolen from a car parked some 15 miles from Birstall in August 2015. But as Mair rarely ventured far from home, and then only on the bus, police were convinced he was not involved in what looked like an opportunistic theft. When forensic tests showed no sign that the weapon had been modified at his house, they concluded that it had been altered before Mair got hold of it.

Whether the gun came first or the idea of murder, by 13 June it seems that Mair’s mind was moving in that direction. That day, back at Batley library, he checked Jo Cox’s Twitter feed. She described herself as “Mum. Proud Yorkshire lass. Labour MP for Batley & Spen. Boat dweller. Mountain climber. Former aid worker.” The day Mair viewed her tweets he also read replies to the question: “Is a .22 round deadly enough to kill with one shot to a human’s head?” His morbid research continued the following day. He searched which parts of the body to target for the utmost harm and looked into a notorious Japanese case of matricide. A poster on the wall of the library advertised Jo Cox’s constituency surgeries and Mair asked a librarian whether he needed to make an appointment or could just turn up.

On 15 June, Mair visited the library closer to his home in Birstall. Staff saw him there often and recalled a man with a slightly florid complexion who did not make conversation or eye contact, who had a favourite computer booth that offered a little more privacy. That afternoon he wore a hooded coat, though it was June, and carried a number of bags including a large holdall. The police thought one might have contained the firearm. Mair spent an hour on the computer. He searched “lying in state”, “lying in repose”, “pauper’s funerals”, “coffins”.

On the morning of 16 June 2016, Jo Cox had appointments in her constituency. As ever, her team were struggling to keep her to a strict schedule as she chatted with the people she met. It was market day in Birstall. CCTV images captured Mair lurking around Market Square, right in the heart of the village, from about midday. From his position, he had a good view of the library a few metres down the street. He was armed with a sawn-off .22 rifle and a dagger. As he waited for the MP to arrive for her 1pm surgery, he ate a chocolate bar.

Jo Cox pulled up with her two assistants about 10 minutes ahead of schedule. As she stepped out on the passenger side, Mair took the gun from beneath his coat and shot her once. When she fell, he dragged her between parked cars and stabbed her 15 times. Bernard Kenny, a 77-year-old former miner, was waiting for his wife outside the library. As he tried to wrestle Mair to the ground, Mair stabbed him in the chest. Mair then shot the MP twice more as she lay on the ground. One bullet passed through her left hand which she had raised in self-defence.

In court, witnesses described the attack on this slight woman as precise and controlled. When one of her aides tried to fend Mair off with her handbag, the MP shouted for both of her assistants to get to safety: “Don’t let him hurt you, let him hurt me.” After the attack, one witness saw Mair walk away calmly, “like he’d done nothing wrong”; another said he showed “no emotion in his face”. About 50 minutes later, almost the same time that Mair was brought down by unarmed police in a suburban cul-de-sac, Jo Cox was pronounced dead at the scene.

Tributes began piling up on the cobblestones of Market Square, beneath the statue of Birstall’s most famous son, Joseph Priestley, the 18th-century scientist and reformer who advocated the open exchange of ideas and toleration. A week later, they were still arriving. The small town was in shock. The murder scene was just a mile from Jo Cox’s childhood home in Heckmondwike. A blanket of white roses, poems and teddy bears spread out beneath Priestley’s feet.

Fieldhead council estate, where Mair lived, is about a mile north of the marketplace. A bright, postwar project, for its first generation of residents it was an escape from the overcrowded slums of Batley and Birstall. It offered people more light and a patch of garden; later, a pub and primary school were added. But over the years, the area was blighted by unemployment, poor educational outcomes, drugs and crime. In 2003, buses stopped running through the estate in the evenings because children would pelt them with stones. Today, there are few shops, no cafe and both the pub and community centre have shut down. Surrounding green spaces have given way to industrial hangars, warehouses and a retail park.

Mair had lived at the same address since the mid-1970s when it was his grandparents’ home, but most people I spoke to said they didn’t know him well. He would occasionally tend the gardens of other elderly residents. Locals saw him as a harmless oddball, out in his anorak with his plastic carrier bags.

Mair was born in Kilmarnock, Ayrshire, in 1963 when his mother Mary was 17. His father worked in the lace industry. The couple’s second child was born three years later, and sometime after that Mair’s parents separated. Around 1970, the family moved south to Batley and the boys lost touch with their father. In the early 1970s Mair’s mother began a relationship with a man from Grenada called Reginald St Louis, and in 1973 they had a son, Duane. Mary lived with her new partner and son in Batley, while Thomas and his brother moved to Fieldhead with their maternal grandparents. Mary and Reginald St Louis eventually married in 1979. The police would state later, in a BBC documentary, that his mother’s relationship with a black man was key to the formation of Mair’s racist views.

I had an exchange of emails with Duane St Louis before he suddenly, and without explanation, stopped responding. He told me that his father found work as a woollen yarn spinner at the Thomas Carr mill. “It was racist in them days in Yorkshire,” Duane remembered, but he said his father was a “big guy” – “6’2”, plenty muscle”, so “they didn’t mess”.

Mair was 15 when his mother married St Louis. Anti-immigrant feeling was high and the National Front (NF), which wanted the repatriation of non-white immigrants, was stirring up unrest with intimidating marches through multiracial towns and cities. It was a time when Margaret Thatcher expressed sympathy with those who worried that Britain would be “swamped by people with a different culture”.

Mary and Reginald had moved on to the Fieldhead estate with Duane after their wedding, and lived just a couple of minutes’ walk from Mair and his grandparents. Duane said he had seen no signs of racism or extremism in his half brother. Friends and neighbours described the relationship between Mair and his mother as attentive and caring. He occasionally took her shopping, did her gardening or went for Sunday lunch. The day before the murder he had been round trying to help tune her television. After Mair’s trial, his mother told ITV News: “I never saw any of this coming.”

But the police sensed that Mair had harboured a deep hostility towards his mother as a result of her relationship with Reginald St Louis; that he felt a sense of betrayal. They implied that he may have intended to harm her after he killed Jo Cox. At the press conference after the trial, Det Supt Wallen pointed out that Mair had reloaded his gun after he shot the MP. A few days before that, he reminded us, Mair had researched the subject of matricide. “Read into that what you will,” he said.

Facebook Twitter Pinterest Thomas Mair. Photograph: SWNS TV

Mair does not appear to have had a job or close friends and lived with his grandmother until her death in July 1996. At 33, Mair was left alone in the house on Fieldhead, the TV in their once-shared living room now turned towards his armchair. These long solitary years apparently took a toll. In her few comments after the murder, his mother said her son had suffered from depression for some 20 years. Mair made few changes to the decor after his grandmother’s death. There were floral rugs and a pink and blue colour scheme. Even the contents of the kitchen cupboards – tinned strawberries, evaporated milk and Rich Tea biscuits – were a throwback to the 1970s. The house seemed lifeless: no pictures on the walls or flowers in the vases, although Mair enjoyed gardening. It was spotlessly clean and ordered – everything had its place, tins carefully lined up, labels facing outwards.

That sense of order extended to the bookshelf in Mair’s bedroom, on top of which he had placed a statue of a Third Reich eagle. It was a well-tended collection almost entirely devoted to Nazism and the second world war. Specialist books such as Belt Buckles and Brocades of the Third Reich must have taken every spare pound Mair, who was unemployed, had. Mair’s library included Holocaust revisionist texts but he also owned Deborah Lipstadt’s Denying the Holocaust, which laid bare the pseudo-science, pseudo-history and antisemitism of such work.

He owned numerous books about race and the evils of miscegenation. SS Race Theory and Mate Selection Guidelines was a compilation of Nazi statements on the importance of racial purity. Another carried an extract from The Turner Diaries, a notorious far-right novel from 1978 which imagined a scene in a future race war: “There are many thousands of hanging female corpses … all wearing identical placards around their neck. They are the White women who were married to or living with Blacks, with Jews, or with other non-white males.”

When the editor of The Press, a Batley and Birstall paper, put out an appeal on my behalf for information about Mair’s life, it drew just two responses. One of them implied Jo Cox’s killing was some kind of pro-remain plot. Birstall seemed anxious to move on, blotting out the memory of a man who had stained the reputation of their community. But one local man with a strong connection to Mair’s family did agree to meet me.

Stephen Lees grew up on Fieldhead estate and was best friends with Mair’s younger brother. He was also witness to the events of 16 June 2016 and gave evidence at the Old Bailey. We met up on a Sunday afternoon, at the Birstall pub that was on Mair’s escape route that day. “I seen Tommy as I were getting off the bus,” about half an hour before the attack, Lees recalled. “I said, ‘Ey up mate!’ and he never said a word.” Despite the men’s long acquaintance, Lees said this was not unusual. “Sometimes he’d say hello, sometimes just nod. It was hard to get a full sentence out of him.”

Lees’s family were also Scottish. His father was a violent alcoholic and left when Lees was 10. At 12, Lees started a fire at his home. “It were a cry for help, I suppose,” he shrugged. He was given a custodial order, and spent a number of years at a community home run by a Catholic order where sexual abuse was rife. It was after his release from custodial care that Lees became friends with Mair’s younger brother, who was then living with his mother. “He was just one of the lads,” Lees told me. “Loud. Fighting when he was younger. Completely different from his brother Tommy.” He remembers being surprised when he met Duane for the first time and he was introduced as “our kid”.

Our conversation was interrupted every so often by friends and family members blundering in from the beer garden. “Go on, lend us a fiver you tight cunt,” Lees’s nephew said at one point, thwacking the back of his head. Lees told me that Mair’s brother was on the edges of the Service Crew, a hooligan firm which attached itself to Leeds United in the 1980s. Black footballers were often abused at Elland Road then, and NF publications and badges with slogans such as “Hitler was a Leeds fan” were openly on sale outside the ground. In June 1989, Lees travelled with his brother and other Fieldhead lads to Dewsbury where a large British National party (BNP) demonstration was taking place. It was the time of the Satanic Verses controversy and the beginning of a period of fraught race relations in the town. The day ended in a “massive fight” between local white youths and Muslims, with most of the BNP contained behind police lines. Despite all this, the far-right sympathies of Lees and his friends was superficial, he said. “It was just lads, you know?” he told me. Lees felt certain that Thomas Mair was not involved in that day’s beer-fuelled hooliganism.

Ecstasy and acid house music soon replaced lager and football for Lees and his Fieldhead friends. Most weekends they would gather at Hartshead Services before setting out on journeys across the north to raves in abandoned warehouses and fields. “No, no, Tommy didn’t get loved up,” Lees laughed. For Lees, though, ecstasy ultimately gave way to heroin addiction and significant terms in prison.

“Tommy looked to me autistic or to have learning difficulties or something,” Lees said. He remembered a time he and Mair’s brother had been to the house, but Mair refused to let them in and began cleaning the garden gate they had touched, using wet-wipes. “He were bad with OCD,” Lees said, “constantly scrubbing his face and hands.” The most he ever heard Mair speak was when he appeared in Birstall and handed his brother a new PlayStation 4. He had spotted it in a bag at the retail park and swiped it.

Now and again others in the pub chipped in with their own memories. Lees’s nephew told me that they called Mair “Marigold” on Fieldhead, because of the gloves he was often seen with for his gardening. A friend mentioned that Mair had been with him on the Priestley wing of Dewsbury hospital. It was the first I had heard that Mair had spent time in secure psychiatric care.

Lees still finds it hard to accept that Mair was capable of murderous violence, describing his friend’s brother as “just a loner and a bit odd”. He was sure, though, that he would not have known how to get hold of a gun. “It’s not easy,” Lees reasoned. “Even I wouldn’t have a clue. Someone must have got into his head.”

At the Old Bailey, where he was called as a witness, Lees remembered Mair facing him in court, staring straight ahead, no recognition in his face. It was as if they were strangers.

Mair was in his mid-20s when he first made contact with far-right groups. It was almost 10 years after his mother’s marriage, and Reginald St Louis had already passed away. In early 1988 Mair wrote to a white supremacist magazine started in South Africa by a British man. Mair had come across the South African publication SA Patriot in Exile through an advert in a National Front publication. The letter referred to the anti-apartheid film Cry Freedom, which Mair said gave “morons another excuse to spout their hatred”. “Despite everything,” he continued, “I still have faith that the White Race will prevail, both in Britain and South Africa, but I fear that it’s going to be a very long and very bloody struggle.”

Mair sent another letter to the SA Patriot, though this one was not published. Dated 2 August 1988, Mair praised the neo-Nazi White Liberation Movement. The group, which sought the removal of black people from South Africa, was banned later that year in the wake of a racist killing. Mair told the editor that he would be “honoured” if his letter were published and that he was happy for his real name to appear “as I have nothing to hide”.

Facebook Twitter Pinterest Books found by police in the home of Thomas Mair. Photograph: West Yorkshire Police/PA

Over two weeks in 1999, a neo-Nazi called David Copeland planted nail bombs at three locations in London, targeting black, Asian and gay people. The most deadly attack, in Soho, killed three. Shortly after the bombings, Mair ordered a number of books which gave instructions on how to make explosives and firearms. Not long after he placed the order, Mair wrote another letter to SA Patriot in Exile, the first for many years. In it, he declared that the greatest enemy of the apartheid system was not black people or the ANC, but “white liberals and traitors”.

Still, apart from such letters, Mair did not surface on the British far-right scene in these years, living a life of quiet anonymity. As he entered middle age, he was stuck in a setting where joblessness and disconnection were common. Fieldhead was, and still is, one of the most deprived wards in the region. In the early 2000s, a project, part EU-funded, was launched on Fieldhead to tackle the problems of social isolation, limited community activity and low self-esteem. When Jo Cox returned to the area as MP in 2015 she found that austerity measures had eaten away the strong sense of solidarity that characterised the place where she had been raised. “I will not live in a country where thousands of people are living lonely lives, forgotten by the rest of us,” she promised.

Though he was lonely, Mair was not entirely adrift. For almost a decade, right up until he murdered Jo Cox, he was enrolled on programmes designed to help those with mental illnesses. His involvement began with a referral to the Pathways day centre, possibly after a period in secure psychiatric care. In 2007, he joined what would become known as the Electronic Village in Dewsbury, a centre teaching IT and other communication skills to improve the job prospects of disabled and disadvantaged people. Most clients were middle-aged and almost half came from the most deprived postcodes of Dewsbury and Batley. Mair spent some nine years learning new skills and then passing them on to others as a voluntary helper.

The centre’s chief executive officer, Karen Wilkinson, remembered Mair as vulnerable and lacking in confidence and got the impression he was being taunted by people on his estate. She said he struggled to make eye contact and rarely smiled. “He would talk to people but was very insecure around them.” She got the impression that he spent Christmas alone. “I think he was very lonely and sad.”

In 2010 Mair became a volunteer gardener at Oakwell Hall Country Park. While he was there, a local reporter interviewed him for a story on the importance of voluntary staff. In a very rare record of his words, Mair told the journalist that the work was helping him to overcome “feelings of worthlessness” and was better than “all the psychotherapy and medication in the world”. He talked of those suffering from mental illness as “socially isolated and disconnected”.

A year later Mair was back at the Electronic Village, this time as a volunteer assistant. For a few hours every Friday afternoon he led a Flexible Learning Group which included a member with schizophrenia and an Asian woman with a history of self-harm. “He had a lot of time for her, he was like a guardian,” Wilkinson recalled. She remembered Mair making balloon animals in one session for light relief. He was generally well thought of although one feedback report hinted at insecurities.

Though Wilkinson never detected far-right sympathies or violent impulses, Mair did not keep his politics entirely under wraps. I met his fellow teaching assistant Tony Kelsall near Dewsbury town hall, where the Electronic Village had been run for a number of years. The son of a Barnsley coal-miner, Kelsall had been knocked off his bicycle at the age of 27 and suffered a severe head injury. The accident left him with problems concentrating and with his short-term memory. Kelsall got to know Mair when they worked together with the Flexible Learning Group. “Tom calmed down some of the quarrels,” he told me. He learned that Mair had received cognitive behaviour therapy for his depression, but felt it had not helped him. Kelsall sensed a caring individual. Mair would always ask about his wife, who has spina bifida.

But Kelsall also saw a political side to his co-worker. Mair expressed frustration about the presence of central European criminal gangs in the UK. Another time Mair became riled when, in a discussion on the American constitution, Kelsall remarked that Thomas Jefferson had fathered children by one of his slaves. The very idea was repugnant to Mair who, in common with white supremacists, hailed the founding father as an opponent of racial mixing.

I was unable to link Mair with far-right organisations operating locally, though in recent times this part of West Yorkshire was fertile ground for the BNP. There was nothing on his bookshelves or his search history to suggest that he was especially agitated by issues that the BNP were exploiting – the building of another mosque, sharia courts, halal slaughter, grooming gangs. “Nobody knew him at all,” David Exley, a Birstall man and a former BNP councillor for Heckmondwike, told me. “He wasn’t a member. He’s never been to one of us meetings.” Gary Stephen, another former BNP member and veteran of violent far-right groups in 1990s Yorkshire, claimed that neither he nor any of his friends encountered Mair. He referred to him as a “bedsit loon”.

Wilkinson told me that she saw Mair for the last time on Friday 10 June 2016, by which time he had the gun and knew what he was going to do with it. His boss saw no warning signs and described her disbelief when she heard about the murder. “It was so far removed from the Tom I know. Isolation, that’s the key to it, I’m sure it is.”

At the Old Bailey, Mr Justice Wilkie described the murder of Jo Cox as both “a personal tragedy and a crime with great public significance”. All seats were full as the trial began, and for one particularly powerful moment during the prosecution’s opening speech, the court fell silent as a 360-degree police video from the immediate aftermath of the killing was played out. It captured debris, blood stains and what looked like a single shoe. As Jo Cox’s parents and sister comforted one another, Mair remained impassive in his seat, detached.

Facebook Twitter Pinterest A memorial event for Jo Cox in Trafalgar Square, London, June 2016. Photograph: Justin Tallis/AFP/Getty Images

Jo Cox’s family did not attend the trial on the day when her injuries were described. “People are killed for what they represent,” a 2013 UN Global Study on Homicide said of socio-political murders, arguing that victims are often anonymous to the perpetrators. But Mair stabbed the MP 15 times, and he had researched where to aim his knife. His attack was so vicious, it seemed motivated by intense personal rage. When Mr Justice Wilkie pointed out that Mair had searched the internet for matricide, knowing that Cox was the mother of two children, her killer screwed up his face as if he had heard something deeply idiotic.

My seat was just a few feet from Mair, who sat motionless through most of the trial. Only after the verdicts when the victim impact statements were read out did Mair reveal the depth of his contempt. Mair was also on trial for the attack on Bernard Kenny, who was still too sick to appear in person. The prosecution counsel read Kenny’s words: “Fear and stress were strangers to me until you sliced a knife into my liver.” Mair was found guilty of grievous bodily harm against Kenny. “I will not think about you again,” Kenny said in his statement, and Mair rolled his eyes towards the ceiling.

Jo Cox’s widower, Brendan, barely glanced at Mair in court as he announced that he did not want retribution. “We have no interest in the perpetrator,” he said in his own, devastating statement. “We feel nothing but pity for him, that his life was so devoid of love that his only way of finding meaning was to attack a defenceless woman who represented the best of our country, in an act of supreme cowardice.” Mair did not react to these words, but as Cox described happy holidays with his wife in Bosnia and Croatia, climbing mountains, abseiling even after she discovered she was pregnant, there was a look of scornful impatience across his face.

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“You are no patriot,” Mr Justice Wilkie declared to Mair in his sentencing remarks. “By your actions you have betrayed the quintessence of our country: its adherence to parliamentary democracy.” Mair’s expression remained blank. But when the judge told him he had also betrayed his parents’ generation, those who made huge sacrifices to defeat Nazism, Mair cast his eyes upwards at the ceiling.

Wilkie gave Mair a whole-life sentence to mark the “exceptional seriousness” of his offence. He would never walk free again. Even then, there was no defiant last outburst. Mair walked from the dock, prison guard on either side, in silence, unsteady on his feet.

After the trial was over, I met with Dr Gwen Adshead, a former forensic psychiatrist at Broadmoor who had worked with Britain’s most dangerous offenders. “You are trying to do what [Mair] refuses to do, to use words to explain something terrible,” Adshead said. She told me such an attempt could only fail. “But that’s all right.”

She felt his crime would have emerged from the slow burn of hostile, murderous feelings nursed over many years. It seemed to the psychiatrist a crime of envy, an act of “motiveless malignity”. And yet, Jo Cox was not a long-studied target. As Adshead pointed out, she had “drifted” into Mair’s consciousness. Mair could have remained a bedsit fascist, flicking through extremist literature alone at home amid his late grandmother’s floral decor. But in June 2016 – with a gun in his hands, at a political moment of great significance – he finally had his “liberal traitor” in his sights.

More than three years after his crime, important questions remain unanswered, including how this apparently isolated individual came to be in possession of a rifle. On Fieldhead, the shutters are now off Mair’s former home in Lowood Lane. A family has moved in, and I heard it had been stripped of its dated decor. The name of Thomas Mair is no longer heard around the estate.

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• This article was amended on 6 December 2019. A reference to Thomas Mair having a shotgun was replaced with rifle, to correspond with the rest of the article.