Six weeks on, there is little to mark the spot on the idyllic rocky beach inlet on Koh Tao where Hannah Witheridge and David Miller met such brutal deaths; just two tiny piles of stones separated by a line of twigs in the sand, someone’s modest, anonymous, temporary memorial.

A few hundred yards away along Sairee beach, the main tourist drag on the Thai holiday island, life continues as normal. Business, says a French man running a dive shop – much of Koh Tao’s tourism is based around diving – is actually busier than expected for the monsoon season. “After the murders you did notice that there were fewer people for a bit. But it was only really the British that stayed away. With everyone else, they didn’t even really notice.”

If this appears curious then Koh Tao, the smallest and most remote among a trio of tourism-dominated islands in the Gulf of Thailand, abounds in such paradoxes.

It is a place where visitors spend their days learning the rigorous safety standards of diving before hopping, without helmets and clad in shorts and vest, on to rickety rental motorbikes. Tourist deaths are not unknown – two bodies of drowned westerners were found in the sea within a couple of days this month – but it is known as one of the safer spots in Thailand.

The biggest contradiction centres around the deaths of Witheridge, 23, and Miller, 24 – the British backpackers brutally beaten on the head yards from their hotel, the former also raped, the latter left to drown in shallow surf. Just about everyone on Koh Tao insists visitors are safe, but many also agree, quietly, that the Burmese migrant workers arrested for the murders are innocent – meaning the real killer or killers remain at large.

Facebook Twitter Pinterest Hannah Witheridge and David Miller were brutally murdered while staying on Koh Tao. Photograph: PA

The island, two hours by boat from the nearest airport, has a low-key, undeveloped feel and mainly attracts younger backpackers. But the ramshackle charm and gentle, palm-dotted beaches are drawing more visitors each year, necessitating new workers.

Many are Burmese, with around 3,000 now on Koh Tao, according to one community leader. “The migrants come here for just one reason – they want a better life, and they’re looking for a job so they can send money back home,” said the man who, like almost everyone else on the island, asked to not be named.

Increasing numbers of Burmese staff the bars and restaurants, in part due to their competent English – an educational legacy of British rule in their home country.

More than 3 million Burmese live in Thailand. As well as low pay and poor conditions, rights groups say, the frequently undocumented migrants face regular and open discrimination, and it is not unknown for police to wrongly blame them for crimes. As the hunt for the Britons’ killers dragged into a third week and the junta’s prime minister, General Prayuth Chan-ocha, said migrants were the most likely culprits, there was understandable nervousness on Koh Tao.

“Shortly after the killings the migrant community started to tell us there was a lot of torture going on, a lot of abuse by the police,” recalled Andy Hall, a British activist who has lived in Thailand for a decade, mainly working with Burmese. “We sent a team down and interviewed the workers, and they were saying, ‘Help us, or they’re going to find a scapegoat.’ And then the Burmese men got arrested.”

Facebook Twitter Pinterest Zaw Lin and Wai Phyo, the two Burmese migrant workers arrested for the murders of Witheridge and Miller. Photograph: Bangkok Post / Barcroft Media

Police said they had confessed, explaining that they found Witheridge and Miller kissing on the beach and, being “aroused”, beat them viciously with a hoe. The case was solved.

Except it was not. Within a few days the men said they had been tortured and withdrew the confessions. Rights groups expressed alarm. The police deny the allegations of torture.

Britain called in a senior Thai diplomat to express its concern. Finally, with deep reluctance and only after David Cameron personally pressured Chan-ocha at a summit in Milan, Thailand agreed to let Metropolitan police detectives review the case.

Now the situation is at an impasse, with prosecutors still awaiting a much-delayed police report. Last Wednesday, a judge remanded the suspects in custody for another 12 days, but warned that without a prosecution they must be freed in little more than a month.

Opinion in Thailand is split. While the government and police chiefs maintain they have their men, others argue the investigation is too obviously motivated by what one Bangkok Post editorial called “rampant ethnic prejudice”.

Among the sceptics is Nakhon Chomphuchat, the leading Thai human rights lawyer defending the suspects. “If I thought they had done it I couldn’t work for them,” he told the Guardian. “Of course, no one can ever say with 100% accuracy, but I’m pretty certain they didn’t.”

Facebook Twitter Pinterest Thai police make alleged killers re-enact murders on Koh Tao beach.

Chomphuchat and others note numerous concerns beyond the confessions. A key part of the police case is DNA evidence supposedly tying the men to the scene. However, Thailand’s most senior pathologist expressed alarm on learning the samples were not collected by trained forensic officers. Meanwhile, CCTV of the key night shows one suspect, Wai Phyo, wearing the same pristine white T-shirt in which he was later arrested.

Chomphuchat and others also stress how unlikely it is for two unworldly young men to commit such a vicious crime and then work as normal for three weeks, even volunteering for DNA tests. In an interview with the Guardian during their court hearing the pair appeared awed and naive, talking excitedly about their love of Manchester United and the thrill of being transferred on a police helicopter.

On Koh Tao, many Thais believe the culprits are Burmese. “A local wouldn’t do it – the locals are friendly,” said a man in his 40s. “But with more development we need migrant workers to help, and these can bring more problems.” Another man, a taxi driver, said: “CCTV shows the men were there, and there’s the DNA tests. Even if we don’t know who the real killer is they’re not Thai, for sure.”

There are several counter theories circulating on Koh Tao about who killed Witheridge and Miller. Most centre around men associated with a dominant Thai family on the island, one of several that run dive schools, resorts and bars. A version recounted repeatedly - without any evidence - is that Witheridge had an argument with one of them at a beachside bar run by the family, shortly before the killings.

The associated media coverage has seen Koh Tao characterised as mafia-run. One English woman, who has lived in the area for many years, says the term is misleading: “The mafia here aren’t the sort who carry guns in violin cases, or knock on doors extorting people. They’re the families that go back for generations, and who ran the islands before the police even got here.”

Facebook Twitter Pinterest Zaw Lin (left) and Wai Phyo have retracted their confessions in the murder case. Photograph: Andy Hall for the Guardian

Nonetheless, there is real fear. Several islanders who believe they know who killed Witheridge and Miller have fled. Those who remain stay silent, for good reason, according to the woman: “If you speak out you’ll suddenly find the landlord of your business doesn’t want to know [you] anymore, or you can’t get a visa. Very occasionally, people disappear.”

The curiosity is that for the vast majority of tourists Koh Tao is safe, at least by the perilous standards of Thailand.

In the last 12 months 362 Britons died in Thailand, more than did so in France, which attracts nearly 20 times more British tourists. This is partly down to Thailand’s younger tourist demographic but also the country’s treacherous roads.

Tourists do fall off motorbikes on Koh Tao – one diving instructor says a student a week is unable to finish a course as they’re swathed in bandages – but the slower, sparser traffic makes very serious incidents less common.

Sairee beach has its rowdy bars along the narrow network of lanes by the beachfront. However, the nightlife culture is tempered by the numbers going diving early in the morning. Drugs are not unknown, but not endemic as on Koh Phangan, the adjoining island famed for its full moon beach parties. Likewise, swimmers do get caught out by currents, but not notably more than elsewhere.

Facebook Twitter Pinterest A police diver searches for clues near the spot where Witheridge and Miller were killed Photograph: Chaiwat Subprasom/Reuters

There is even a local theory that the dominant families actually keep Koh Tao more secure – they have become fantastically rich through tourism and take a dim view of crime. “I’ve never felt as safe living anywhere,” said one young Canadian woman based on the island.

This is what makes the murders of Witheridge and Miller simultaneously so exceptional and difficult to resolve. If the theories are correct, and the real killers are wayward elements of a dominant family, their wealth and hold over police makes it very unlikely they will face charges.

At the centre of this impasse – and facing possible execution by lethal injection – are Zaw Lin and Wai Phyo. However much the evidence might tip in their favour, to not prosecute would mean acknowledging that a high-profile investigation endorsed by the junta was flawed.

Equally central, and in danger of being forgotten amid their current silence, are the grieving families of Witheridge and Miller, who have yet to express a view.

Last week lawyers for Zaw Lin and Wai Phyo passed a handwritten letter from the pair to the Guardian, appealing to the British families to share any information that could spare them.

A key moment will come with the completion of the Met’s report, which will be shown first to the grieving relatives. They have so far ignored their right under Thai law to participate in the judicial process.

Amid the maze of contradictions and powerful vested interests, one of the few optimistic voices comes from Zaw Lin, usually the less buoyant of the arrested pair. Asked by the Guardian how he judged his fate, he replied: “There are so many people helping us, we are sure we can get justice. And we didn’t do it.”