He was one of B-wing’s most popular characters, although recently other detainees had noticed a sudden darkening in his mood. A little before 5pm on 3 September, two days before his 28th birthday, Marcin Gwozdzinski made a final plea for help. Addressing officials inside the administrative quarters of Harmondsworth immigration removal centre, he told them he could no longer cope.

“He was crying, begging for help from the guards, telling them to call an ambulance, that his mental health was an emergency,” said another detainee, Marcin Malicki, who sat beside Gwozdzinski in the meeting. “They told him he would get no help and to stop calling for an ambulance,” said Malicki, 37, who was there as a translator. “He broke down like a baby. Still they did nothing.”

Shortly after 7pm, Gwozdzinski asked if he could borrow a Qur’an, a request Malicki thought was peculiar from a devout Catholic, the latest clue, perhaps, that his friend was losing his mind. Other detainees wondered why he was not on suicide watch, or at least antidepressants.

At 2.05pm the next day Malicki heard shrieking from inmates huddled outside the door of room 10, B-wing – Gwozdzinski’s cell. Malicki found his friend barely alive. He tried to revive him. “I still carry his face in my head, all the time I see him,” he recalls.

Gwozdzinski’s was one of three deaths linked to the UK’s immigration detention estate in the space of a month. This year is the deadliest on record – 10 immigration detainees have died, twice the previous high. News of the latest emerged two weeks ago – another suicide, a 27-year-old Iraqi man inside Lincolnshire’s Morton Hall detention facility. Few other details have surfaced, but critics say such secrecy is the norm.

Of the 10 deaths, three of their names are not known. Occasionally, the Home Office fails even to publicise a death. One recent Freedom of Information response found two deaths of which campaigners had no previous knowledge. In keeping with such a clandestine climate, the Home Office has not released Gwozdzinski’s name, or even his age. Until now, who he was or the circumstances of his death were a mystery.

Gwozdzinski’s family claim they are being kept in the dark, that his death was preventable but that its circumstances are being covered up. Beyond dispute is that the tragedy arrives during a particularly contentious period for the UK’s immigration detention estate, specifically its ongoing role in the detention and deportation of European Union citizens from the UK following the Brexit vote.

Home Office data released on Thursday confirmed 5,321 EU nationals were forcibly removed in the year until September, a 13% increase on statistics already at their highest since records began.

Celia Clarke, director of the charity Bail for Immigration Detainees believes Brexit has given the Home Office the green light to target Europeans in the UK, an allegation corroborated by claims that EU citizens are being detained for relatively minor crimes, such as driving offences. “If you have a government that unashamedly vows to create a ‘hostile environment’ then it is no surprise that people are swept up, detained and removed, with little regard to their rights,” said Clarke. Her organisation is fighting the deportation of 70 EU nationals – two thirds of whom have no criminal record and whose average stay in the UK is 11.4 years.

Also on Thursday, the former Tory international development secretary Andrew Mitchell became the latest public figure to voice concern over the purpose of the UK’s 11 immigration centres, denouncing them as a “dystopian stain on our democracy”.

Another dimension is unfolding that is driving further disquiet: the allegedly systematic clearance of homeless EU nationals from British streets. The numbers affected are unclear – despite FoI requests, the Home Office refuses to publish updated numbers – but Clarke believes the amount of Europeans targeted under this policy may roll into the thousands. A high court judge will rule next month whether the policy is lawful. Gwozdzinski and Malicki were both homeless when detained. More are being swept up all the time.

The edict has caused consternation among diplomats. Vali Staicu, third secretary at the Romanian embassy, said her country had raised the issue with the home secretary Amber Rudd last month. “We are very concerned, we have cases where the Home Office say it’s a voluntary return but when we ask, the individual says, ‘I don’t want to go.’”

The Slovak embassy was worried about people being detained and deported without “legitimate reason”.

Sources at the Polish embassy, which represents 831,000 Polish-born UK residents, making Poles the largest overseas-born group in the country, would only confirm “cooperation” with the Home Office on the issue.

Born in the remote town of Cibórz, near Świebodzinin west Poland, Gwozdzinski – the youngest of seven brothers – was an adventurous child. As a boy he explored the surrounding spruce forests, travelling across Poland as a teenager, then Europe, Ireland, eventually arriving in the UK aged 23. Travel, he told his family, helped “keep his spirit free”.

His brother Grzegorz, 30, said: “He loved new countries, new people, and although he was a hard worker he enjoyed sleeping rough – squats, empty buildings, anywhere.”

In October last year and despite working on a building site in Tooting, south London, Gwozdzinski had elected to sleep rough near Streatham Common. He smoked cannabis to help him sleep and one evening approached dealers near Streatham’s St Leonard’s Church to stock up on supplies. Gwozdzinski was short-changed, a scuffle broke out that was caught on CCTV. The following morning, he was arrested and taken to Harmondsworth. Quick witted and gregarious, he was quickly dubbed by B-wing as Siwy, which translates as light blond in Polish. He had no history of mental illness and was generally happy. His family had never contemplated that he might one day try to kill himself.

Three months on and no official account into the events around his death has been issued. The Prisons and Probation Ombudsman (PPO) is investigating but will not comment, while the Home Office says it is “inappropriate” to share statements on an ongoing PPO inquiry. Mitie, the private firm that runs Harmondsworth on behalf of the Home Office, declined to answer questions. However, corroborating witness statements indicate that Gwozdzinski begged for help repeatedly over a period of time. Another Polish detainee on B-wing, Robert Legut, 52, describes Gwozdzinski telling staff on several occasions that his mental health was deteriorating profoundly.

“He was saying: ‘Help me please, my head is no good. I might do something, suicidal.’ He told guards five days beforehand, two or three times. They ignored him.”

Campaigners in London protest against immigration detention. Photograph: Alamy Stock Photo

Speaking in the spartan, heavily securitised meeting room inside Harmondsworth, the rumble of flights from nearby Heathrow airport in the background, Legut said Gwozdzinski’s death was avoidable. “He went back to get help but they laughed at him again. He was desperate, why did they not give him a psychologist? His death is the Home Office’s fault.”

A written statement, signed by 59 fellow detainees of various nationalities, unequivocally blames the death of their “friend” on the authorities. “For a long time he [Gwozdzinski] asked officers, psychologists and doctors for help. He was ignored,” states the carefully handwritten letter. “Many times he asked for help. Therefore myself and other detainees are very anxious and depressed about the situation. It is a disgrace that nobody has been [made] accountable for such poor care. We are human beings not animals.”

In a letter written by Gwozdzinski shortly before his death, he says: “I can’t be here. I want to be free … there is not much time left.”

The partner of Gwozdzinski’s brother, Ewa Jany, says detainees have described how Gwozdzinski’s roommate would stand over him at night, reciting Satanic verses. “We heard how his cellmate would stand over Marcin at night talking about witches and Satan. Marcin was having terrible nightmares, it is no surprise.”

Days before Gwozdzinski tried to kill himself, he asked the centre’s authorities to be moved. But Jany believes it was too late: the damage had been done. “He needed special supervision, there are staff who follow inmates with psychological problems. They even follow them to the toilet. Why did Marcin get no one?”

Wider, structural issues may have played a part. Britain remains the only country in Europe with no time limit on immigration detention. With detainees’ lives in limbo, psychologists agree that the system affects mental health. Two years ago MPs joined campaigners in urging a 28-day limit on immigration detention, and only then as an “absolute last resort”.

Legut, who says he was detained for minor motoring offences, has been held since January. Malicki was held for nearly a year, the same as Gwozdzinski. All three registered mental health complaints, but none was given antidepressants or psychological assistance.

When the Observer met Legut inside Harmondsworth, his arms were covered in a blood-encrusted latticework of self-inflicted cuts. During our conversation, Legut, who has lived in Scotland for 12 years, ran a finger across his throat to mimic a neighbouring detainee who had cut his throat days earlier. Legut texted twice during the following three weeks with details of others who had self-harmed, and these were just among his 14 Polish friends.

Suicide attempts are rising across the immigration detention estate. According to data from campaigners, 132 attempts were recorded during the first quarter of this year, a rate that could eclipse the record of 393 in 2015. Legut believes the rise is bolstered by the prevalence of the synthetic drug spice, linked to psychotic episodes and erratic behaviour.

Although spice’s ubiquity inside prisons is widely chronicled, its spread inside immigration centres is not. Malicki says spice was easily obtainable in Harmondsworth and that Gwozdzinski was a user, although its influence in his decline is ambiguous. “I asked staff why they were letting so much Spice inside – people were going crazy – but they said, ‘You stop one dealer and another will take their place.’”

Emma Ginn, coordinator of Medical Justice, which works to improve the health of detainees, said her organisation had frequently warned the Home Office that conditions inside would lead to further deaths if not addressed. “Clients call us, petrified that they too might die,” she said. “They say the authorities don’t care whether they live or die. We can’t console them.” Deborah Coles, director of Inquest, said: “Deaths and self-harm are the human consequences of the UK’s dehumanising and unjust detention system.”

Gwozdzinski’s seemingly swift decline has been difficult for his family to digest. He had been talking about his future – accepting that he might need some stability according to his brother Grzegorz – but was also in peak physical condition. When his mother, who lost another son in a car accident in Poland seven years earlier, accepted medical advice and turned off her youngest’s life support machine at 6.40pm on 7 September, Gwozdzinski’s heart kept beating unaided for 25 minutes. “Two nurses were there and they were very surprised. His heart must have been so strong, but the brain was too damaged,” said Jany.

Since then, the family allege attempts to work out what happened inside Harmondsworth have been obstructed. After Gwozdzinski’s death, key witnesses were deported. Despite nearly 12 months in detention, the prime witness Malicki was sent to Poland within days. Weeks later, when the Observer finally tracked him down, he was in Iceland. Legut said four witnesses had been deported or “ghosted” to other centres. A month after Gwozdzinski’s death, Legut revealed how one witness was flown to Poland at 8am, two hours before a case hearing. “It was too fast a decision to send them away; they are silenced,” he said. Neither Malicki nor Legut have been interviewed by investigators charged with finding out what happened.

There are also allegations that the Home Office attempted to keep Gwozdzinski’s suicide bid quiet. Four hours after the attempt, at 6pm on Sunday, Jany and Grzegorz were notified he was in hospital. Throughout the following day, the Home Office refused to confirm claims that a detainee was on a life support machine. That night the BBC aired a damning documentary showing guards at another removal centre mistreating vulnerable people, including those who were suicidal. Another four days would pass before the Home Office confirmed something had happened. Detainees were sent a note citing an “incident” involving a “Polish detainee” who had died. Legut was among those kept in the dark. “They didn’t even tell his friends.”

The sense that the Home Office is covering up the deaths of other EU nationals is shared. Eight months after Slovenian Branko Zdravkovic, 43, killed himself in a Dorset immigration detention centre, his partner, Nicola Sanderson, says she is still waiting for answers, but believes he was not given the care he needed.

Zdravkovic, she says, was detained following a sweep of east Europeans drinking on a street in central London. Once inside, his wellbeing disintegrated. “They didn’t contact me at all, it’s disgraceful. My Branko was born in a quiet village near Austria and comes to this country, works for years, and this is how he was treated,” said Sanderson, from west London.

Other aspects of Gwozdzinski’s case have prompted unease. Two days after his suicide attempt, Jany received a call from Harmondsworth informing her that Gwozdzinski was to be released. The official explained, says Jany, that he was no longer considered a threat to the community. “I actually started laughing, it was so ridiculous. Marcin was brain-damaged in a coma.”

Another twist emerges with claims that staff apparently told Gwozdzinski that weeks before his death he was to be released. “The Home Office told him he was free to go. But they didn’t release him,” said Legut.

Jany said they were actually preparing for his homecoming. “What was odd is that Marcin called and asked for our address. He said the Home Office would release him if he could provide an address that they could then check.”

For three weeks Gwozdzinski called Jany daily, eagerly inquiring if the Home Office had called. “We were ready for them to check our house but nothing happened. I think this eventually broke him down.”

Such episodes have left the Gwozdzinskis asking if the death of Siwy, a free spirit crushed by detention, can be tied to Theresa May’s ongoing “hostile environment” against those she believes should not be in the country.

Jany said: “The Home Office behaves as if it is scared of Polish people. We need to know why Marcin was detained for so long, why he hanged himself.”

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