On a sun-bleached street in Mandeville, a cluttered town in the deep velvet of Jamaica’s green hills, the Sunday-morning silence dissolves slowly. A mixtape of reggae songs begins to play for the large crowd gathering in the heat and Karla Mohammed, a 43-year old British woman, leans into the open boot of a white Escalade, weeping.

She has the bewildered look of someone unsure how she came to be in this bauxite boom-town in Jamaica, burying her 18-year-old boy. Ever since Mzee Mohammed died on 13 July in Liverpool, in circumstances that remain mysterious, she has navigated plans for two funerals – one in Liverpool and a second in Jamaica – an Independent Police Complaints Commission (IPCC) investigation, an inquest, and the puzzling bureaucracy of reclaiming a child’s body from the state. It hasn’t been easy, far from it, and even now, as family members gather around the hearse, there is talk of the smell from the corpse.

There were eight security guards, 18 police and a helicopter. All for one lad who was already on the ground

“First they killed him, and then they gave the family a rotting body to bring back here. It’s just very unfortunate,” mutters Karen Williams, a cousin of Mzee’s who has travelled from Philadelphia. “Even after he was dying, Mzee was still not being treated as if he was human. He died in July, they were doing different tests, taking his body in and out, it wasn’t handled well. People can be so cruel.”

It’s one of many grievances shared by the family – a large Jamaican clan who have gathered in Mandeville, their ancestral town, from diaspora homes in Toronto, London, Liverpool, New York, Philadelphia and Orlando. They had to fight to find someone willing to embalm a body already in a state of decay. “In our culture, cremation is not the biblical way,” says Joanna Smith, a cousin of Mzee’s from Washington DC who speaks rapidly and authoritatively. “To get Mzee’s body here to be buried was already a victory.” There is a sense that this is a family that has been plunged into a battle they are only just beginning to comprehend.

Cut short: Mzee Mohammed died after being detained by police at the Liverpool ONE shopping centre on 13 July 2016. Photograph: Liverpool Echo

It all began on 13 July at 12:30pm when Mzee Mohammed, a sometime college student who loved his food, headed to his father’s house in the hope of some Jamaican cooking, to “pree his pot” in his family’s Liverpool-Jamaican slang. Although Mzee lived with Karla, his half-Kenyan, half-English, Liverpool-born mother, he was closest to his father, Clement “Blacks” Daley.

How could we have known that the next Black Lives Matter march would be for him?

It was Karla’s bingo night and Mzee promised to be home by 6:30pm to babysit his two younger siblings. Mzee suffered from autism and ADHD, and had avoided crowds since childhood. But after what relatives describe as a racially motivated attack two years earlier had left him hospitalised, he began travelling by taxi. “If he had to buy something in town, he would have to know beforehand what shop, and go straight there,” said his cousin Kalum Wrigley. “He was autistic, but he was functioning. He knew what his limitations were. That’s why it doesn’t make any sense for him to go into town and just be running around.”

Yet that’s exactly what Mzee seems to have done. At some point in the afternoon, he’d left his father’s house and gone into a nearby shop in a highly agitated state, claiming he was being chased. The shop owner, who’d known Mzee all his life, called a car to take him home. But instead, Mzee ended up at Liverpool’s main shopping centre, Liverpool One. By coincidence, Wrigley – who works as a supervisor at the Subway fast food outlet inside the centre – began following events in real time, when he heard security guards discussing an incident on the internal comms.

“We heard the security saying there was a ‘big black male with dreads running around barefoot with a knife’,” Wrigley said, echoing initial press reports that Mzee had been threatening shoppers with a 12in blade. Those claims have never been confirmed, but Wrigley was concerned enough that he asked his manager whether he should close up the shop until the incident was over.

Last moments: a still taken from witness footage of Mzee Mohammed being detained in Liverpool.

The family accept that something was wrong. But the nature of the police response, they believe, was disproportionate, and ultimately lethal. “I heard the command for the security guards to call the police, and at that point they already had the person detained,” said Wrigley. “Knowing that, they still sent 18 police officers. So there were eight security guards, 18 police officers, a police dog and a helicopter. All for one lad who was already handcuffed and on the ground.”

It’s an image that was broadcast to the world, after an onlooker filmed Mzee, lying face down as officers stood and kneeled around him, his hands bound behind his back. And then an ambulance was called, and at 7:53pm he was pronounced dead. An IPCC investigation was opened shortly after.

Mzee’s case was immediately taken up by campaigners concerned about the nature of black deaths following contact with the police. “There is a real issue with the disproportionate number of black people who die following the use of force after contact with the police,” said Deborah Coles of Inquest, which helps families affected by deaths in state custody. “They are the most contentious deaths, they have raised serious public and political disquiet, and concerns about racial profiling and the stereotyping of, in particular, young black men as being ‘big, black and dangerous’.”

The extent to which black people are disproportionately represented in deaths at the hands of the state is the subject of scrutiny. Figures from Inquest show that one in seven deaths in custody and one in five deaths after being shot by police involved people from minority backgrounds, who make up one-tenth of the population as a whole. IPCC statistics reveal that over the past decade, 6% of those who died in police custody were black, compared to their 4% representation in the population at large. On the other hand, some caution that the numbers of people dying are too small to draw big, statical conclusions. Although 1,500 have died following police contact since 1990, the frequency of those deaths has fallen over time. Britain is not America.

Taking to the streets: a Black Lives Matter demonstration for Mzee Mohammed in London. Marcia Rigg holds a placard – her brother Sean also died in custody, in 2008. Photograph: David Mbiyu/Getty Images

But because of the figures, and infamous individual cases – such as Sean Rigg, whose death at Brixton police station in 2008 remains a source of protest – and a wider sense of racial injustice in the criminal justice system, Black Lives Matter has been gaining ground in the UK. On 10 July, just three days before Mzee died, it held its first protest in Liverpool. At the time, Mzee asked his mother if she was going, and when she showed little enthusiasm, made fun of her for being more interested in bingo. “We laughed about it,” Karla says, a bewildered expression on her face. “How could we have known that the next Black Lives Matter march in Liverpool would be for him?”

The weekend after Mzee’s death, several hundred friends, family members and activists marched through Liverpool demanding answers. In August, a coroner formally opened the inquest, berating the IPCC for the slow pace of its investigation.

It’s a claim that the IPCC deny, arguing that they have been working within the relevant timeframe. “The IPCC has prioritised securing relevant CCTV and then has spoken to a large number of witnesses in priority order, in line with our witness strategy,” a spokesperson said. Merseyside Police said they were cooperating fully with the IPCC and wouldn’t comment on any aspects of the case to “preserve the investigation’s independence and integrity”.

That is not a message that goes down well in Mandeville, where “Merseyside Police” and “IPCC” are mentioned so often they have become unwelcome guests at the funeral. “Lying Lips is an Abomination,” is the slogan on T-shirts worn by the freckled young men from Liverpool who haul the coffin from the Escalade into the sweaty shade of a large Seventh Day Adventist church. The funeral service is part fire and brimstone – “No one knows what that young man saw when death looked him in the face!” booms the preacher. “Perhaps he asked for forgiveness!” – but it is also protest. One after the other, aunts, uncles, cousins and friends testify to the good of Mzee’s infectious smile, and the wrongness of his death. “You died handcuffed on the floor like a slave,” declares Roxanne Tagoe, a family friend whose poetic tribute to Mzee draws nods of approval during the funeral. “I make you a promise, we will fight to the end. We will find out who killed you, my friend.”

Spreading the word: Black Lives Matter protesters on Oxford Street. Photograph: David Mbiyu/Getty Images

“Over the centuries, our people have been marginalised by systems that have been orchestrated by racism!” another relative reads. “Slavery and racism are demons. How many of us have experienced racism first hand?”

It’s a question that resonates. The family trace their lineage back to that era of slavery in Jamaica. “This land where we are burying Mzee was inherited from my great, great grandfather, who had been a slave,” said Mzee’s aunt, Carmen Hutchinson. “When he was freed, he cultivated all these lands: coffee, oranges, pimento, potato and yam.” Two centuries later, traces of slavery permeate all aspects of life. The night before Mzee’s funeral, family and friends gathered for his wake, under a night sky made smokey by the jerk chicken barbecue, a ritual that some call “Nine Night”, a name handed down by African slaves who believed it took nine days for the deceased’s spirit to return home to Africa.

Slavery ended in Jamaica in 1833, but its abolition was the start of a poverty that has persisted, prompting Jamaicans to migrate abroad in search of a better life. So many have left that Jamaica’s diaspora is now at least as big as its home population: nearly 2m Jamaicans live in the US; 650,000 in the UK. A greater number of second and third generation descendants are British, American and Canadian citizens.

As a result, the question of what happens to Jamaicans when they leave the island is a sensitive one. “Our children leave Jamaica to prosper,” said Hutchinson, two of whose daughters have emigrated to the US. “It’s the best thing to do: the opportunities are there. But the racism… I’m worried about our young men.”

Mandeville has become known as a centre for “returnees” – Jamaicans in the diaspora who have decided to return home, disillusioned with life abroad and attracted by the availability of land and a crime rate that’s lower than Kingston and Montego Bay.

Mzee was hoping to be among them. In recent years he had become increasingly intent on emigrating to Jamaica. That fact, well known among all who knew him, only deepens the sorrow of those walking alongside his coffin on its last journey, up a steep path behind his 90-year-old grandmother’s house, past the lines of washing and the rainwater pool for watering vegetables. A newly cast white concrete tomb, painfully bright in the midday sun, slowly disappears beneath a crowd of mourners, and the women begin singing spirituals: “No Grave Can Hold My Body Down.”

For his mother Karla, the final frontier between her and abject grief falls away. As the coffin is pushed gently into the tomb, her low wailing is the most powerful sound.