The attack on Professor Rezaul Karim Siddiquee was so frenzied that its traces remain more than a month later, arcs of dried blood spattered up a pink wall and a pile of sand covering bloodstains that had pooled on the ground where the softly spoken lecturer was all but beheaded.

He was killed on his way to work in the city of Rajshahi by four men who knew their target and his routines well. At least one of the killers was a former student who had a reputation for barracking the professor in class about the “immorality” of the English literature he taught, police believe.

Neighbours in the narrow alley where Siddiquee was murdered overheard him greet someone moments before his death. “You’re here?” he asked his killer. His final words were spoken in surprise but not fear, because Siddiquee never imagined that he would be a target for extremists, his family says.

The murder fitted into a pattern laid down over a gruesome three-year killing spree by extremist groups in Bangladesh: a bloody but brutal attack in broad daylight with the most basic of weapons, and later a claim of responsibility from Islamic State (Isis) or al-Qaida.

But it was also a warning of the way the killers have expanded their campaign, from a focused assault on secular activists into a wider war to reshape Bangladeshi society along lines determined by Islamist extremists.

Siddiquee was an observant believer who regularly attended prayers and even paid for the renovation of the mosque in his ancestral village – his was the most anodyne of public profiles. If he was a target, surely millions of other Bangladeshis are too.

“What he was, we all are. If a person like him who loves to read, recite literature and play music at home can be killed, all of us are liable to be next,” said one of Siddiquee’s colleagues, asking not to be named for fear it could push him up a list of possible targets.

Foreigners, religious minorities from Hindus to Christians, Muslims from other sects and even Sunnis who subscribe to a more generous vision of faith than their attackers, are all now at risk.

Since 2013, 30 people have been murdered. They were from all faiths and social backgrounds, linked above all by the manner of their deaths, at the hands of men wielding machetes, knives and even swords. At least three others barely escaped assassination attempts, surviving with scars on their faces and necks that look like medieval battle injuries.

One, Ahmed Rahim Tutul (pictured on page 15), survived only because he fell between a table and chair as a militant slashed at his head with a sword in an attack that left terrible scars across his face.

Ahmed Rahim Tutul. Photograph: Ismail Ferdous/Redux/Eyevine

The toll is tiny in absolute terms for a country of around 160 million people, and authorities insist they have the upper hand in the battle against what they describe as a relatively small, unprofessional band of fanatics, pointing to dozens of arrests. On Saturday, after the latest killing, security forces rounded up 1,600 people in a show of strength, although they admitted that only 37 of the detained were suspected Islamist militants, and the rest were petty criminals, AP reported.

“If they think they could turn Bangladesh upside down, they are wrong,” prime minister Sheikh Hasina told parliament before the raids.

But the brutality has thrown daily life out of kilter in disturbing ways, pushing the country towards the conservatism and religious monoculture that the attackers apparently seek.

At the country’s second-largest university, where Siddiquee taught, professors are curtailing classes out of fear of their own students. Authorities have received a hit list of around 40 professors, and some have received threats by phone and letter.

A week after Siddiquee’s killing, three men on a motorbike roared into the village of a Hindu tailor, Nikhil Joarder, hacked him to death and threw his body in a ditch. Again, they struck in the middle of the day, on a main road lined with shops and homes, but his former friends and neighbours all insist that no one saw the faces of his killers.

The murder put an end not just to Joarder’s life, but to a long history of religious diversity in the village. Joarder’s wife and his brother’s family fled after the killing, and now the courtyard of corrugated iron homes that was the tiny Hindu enclave is locked and empty.

“I came here for my security. I have nothing now,” said his widow Aruti Rani Joarder, weeping at a relative’s home in a nearby town. “This is not a safe country for the Hindu community.”

Despite the government’s promise of swift justice and a string of arrests such attacks have, if anything, gathered pace. So far in June, four people have been murdered: two Hindus, a Christian trader and the wife of a senior police officer tasked with stopping militants, a cross-section of Bangladeshi society. As the list of victims has lengthened, so has the sense of menace across the country.

The first attacks targeted only prominent secular intellectuals, a tiny and easily identified group. Few people are ready to take a prominent public stand against religion in a country where Islamist groups have deep roots and where there is a devout Muslim majority.

After killers circulated the bloggers’ work beyond its original audience, outrage dimmed to apathy among many people, who considered the attacks reasonable punishment for what conservative Islam deems a capital crime.The nominally secular government did little to dispel the impression that the killings were disturbing but a minority concern. Hasina’s government is waging a bitter political battle against Islamist and conservative opposition groups, and is apparently unwilling to risk popular support with a fierce defence of unpopular radicals.

Her government’s muted response to killings has been laced with insinuations that the bloggers contributed at least in part to their murders. “These attacks are not acceptable, but at the same time we expect people to stop criticising the prophet Muhammad,” said Shahriar Alam, one of the country’s junior foreign ministers, told the Observer.

But the bloggers are now a minority among the dead, rendering pointless the calls for self-censorship. Spiritual leaders, foreign aid workers and ordinary members of minority religions appear to have been targeted more for what they represent than anything they have done, making the murders impossible to predict or prevent.

Students and teachers march through the campus of Rajshahi university to protest the murder of English professor Rezaul Karim Siddiquee. Photograph: Emma Graham-Harrison/The Observer

UNCLAIMED KILLINGS

One of the peculiarities of the killings in Bangladesh is that none has been claimed by a local operation. There have been no demands or ultimatums, or any explanation for why victims were targeted.

That vacuum has been filled only by statements from Isis and al-Qaida, claiming responsibility for killings thousands of miles from their nearest known base, and made in Arabic, a language not widely spoken in Bangladesh except by religious scholars. A recent Isis propaganda magazine boasted an interview with a man whom they claimed was head of operations in Bangladesh.

Analysts say it is extremely unlikely that Isis has set up a cell in Bangladesh. “It’s physically impossible for an organisation that is Iraq- or Syria-based to go somewhere so far away and launch operations,” said Kamran Bokhari, a fellow at George Washington University’s programme on extremism and an expert on South Asia.

But the consistency and speed of the statements suggest there is an Isis link with killers in Bangladesh, perhaps through a member of the diaspora, turning the attacks into a propaganda tool for both parties.

For local organisations, even a basic link-up with Isis brings extra publicity and possibly recruits, as well as the potential of hard cash. “You go from relative obscurity to an influx of tradecraft, maybe cash.”

Such a connection may be deepened if the government cannot stop the attacks, as local groups gain a reputation and improve their operational skills. “If this cell doesn’t die down, how long are they just going to use machetes? They are going to get more confident, more emboldened,” Bokhari said.

Hasina, who has insisted that the killings are part of a political campaign by local terror groups and fiercely rejects the possibility that al-Qaida or Isis have a foothold in Bangladesh, promised a stronger response after the latest attack.

The man charged with responding will be police commissioner Monirul Islam, who broke up the cell behind some of the earliest attacks on bloggers and now heads a 600-man counter-terror police unit and says he is chasing two groups. The first is a sophisticated and well-financed operation that targets bloggers and secular activists in carefully planned attacks – a group once known as Ansarullah Bangla Team but recently renamed Ansar al Islam.

Members arrested in connection with attacks include the son of a top banking executive, the nephew of a deputy minister, and a man who worked for multinationals including Coca-Cola. Commissioner Islam believes the group is still dangerous, and considers cracking it one of his biggest tasks. “They are the real extremist group: they follow the ideology of some global outfit, like al-Qaida, though they don’t have direct connections with them,” he said.

The second network is responsible for most of the recent killings, Islam believes. A re-formed wing of local terror group Jaamat-ul-Mujahideen Bangladesh (JMB), which set off bombs around Bangladesh a decade ago before being largely dismantled, it is now targeting Hindus, Christans, foreigners and Buddhists to send a political message.

Rizwana Hasin and Rissat Imtiaz, the children of murdered English professor Rezaul Karim Siddiquee. Photograph: Emma Graham-Harrison/The Observer

“They want to prove the government is incapable of protecting minorities,” Islam said.

They are less sophisticated and less well-financed than the group pursuing bloggers, Islam says, which should make the killings easier to stop. At present, he believes, the group has just a few dozen members, and many have been rounded up already, or are on a “most-wanted” list.

Among them are the men behind Professor Siddiquee’s murder, former students frustrated by classes on literature which they considered transgressive.

“Two of his students were involved in this killing, former honours students,” said Islam. “One was very vocal in class, sometimes used to oppose the professor’s views, particularly during discussions on fiction and non-fiction. Eventually, he dropped out.”

POLICE UNDER PRESSURE

Even those most desperate for police to find the killers are unsure if the people they are arresting are the right ones. Siddiquee’s family and former colleagues say they want real justice, not simply detentions with few details and a legal process so glacially slow that the men accused of killing another professor in 2004 only came to trial this year.

Analysts say there is a risk that police, who were criticised for brutality in the past, and who are relatively unfamiliar with tackling extremist cells, could resort to indiscriminate tactics. “They go around shopping people, beating people up – and they create further resentment in society by targeting the wrong people – there are lots of ideological extremists who aren’t terrorists,” said Bokhari.

There is already concern about the death of a young student in custody, after he was arrested in connection with Siddiquee’s murder. Shortly after the killing, officers swooped on a working-class home near the murder site and picked up Mohammad Hafizur Rahman. He died in jail a few weeks later, and the family is still waiting for his death certificate.

The first person in his family to go to university, Rahman was a second-year student in public administration, whose parents say he was devout but not militant. “He was the big hope of our family,” said Halima Begum, his mother, as she leafed tearfully through his graduation certificates from schools and madrassas, all boasting top grades.

Halima Begum sits beside the books and desk of her son Muhammad Hafizur Rahman. Photograph: Emma Graham-Harrison/The Observer

They believe that neighbours falsely accused him of a role in the killing as part of a feud, and they say Rahman was ready to wait out a process which he assured them would end with his innocence being vindicated.

He suffered from the blood disorder thalassaemia, and police said he died from natural causes, although officers struggled to explain the lack of a death certificate.

“It’s not what people say, a death in police custody; he was well taken care of by the doctors in the jail hospital,” said local police chief Sardar Tamizuddin Ahmed, who added that Rahman had not been a key suspect. “It’s not that he was involved, but there are sometimes many sorts of information that help investigating the case.”

His relatives are careful to say that they do not blame the police for his death, but they insist he had all the medicines he needed for his condition. They also say there were large unexplained welts on his waist when his body was returned to the family, and they question why they are still waiting for a death certificate.

“Each day we ask for it and they give us a different reason not to issue it. They know my brother is innocent, he has no crimes committed, that’s why they are delaying,” said Habib Rahman. In an indication of official attitudes to police brutality, minister Alam said he had no concerns about either the death, or the potential loss of a key witness.

HISTORY OF RADICALISM

Tensions are now so high in Rajshahi that police wait at the airport to offer permanent armed escorts to any foreigners flying into town.

On a sunset stroll beside the river Padma, Rajashi hardly seems a threatening place: friends, families and couples gather to gossip and take endless selfies, watching river dolphins tumble through the muddy waters and snacking on fruit.

But barely a kilometre away, the blood stains in the alley where Professor Siddiquee was murdered are testament to a much darker side of the city – and a disturbing history of extremism – that authorities have been unable or unwilling to tackle.

Its activists are so committed, according to police and other officials, that decades ago they ensured that they married into families based near the university to secure a long-term base.

Such historical allegations are difficult to verify, but one of the many unsettling facts about Siddiquee’s death is that it was not the first such killing: he was the fourth faculty member of Rajshahi university, one of the most prestigious in Bangladesh, to be murdered by extremists within a decade. In at least two other cases, student suspects were caught.

“When they choose a victim, they choose always a person who is involved in a large community, [who is] educated and has a strong conscience,” said Ahmed, the Rajshahi police commissioner. “A teacher is an easy target for them, and when a teacher is killed there is a very big outcry.”

Although individual terror raids or mass shootings have claimed more lives at schools and colleges in other countries over that period, and universities often become battlegrounds when nations go to war, for a country at peace, the rate of extremist assassinations – and authorities’ failure to stop them – is staggering.

However, at the university there is widespread denial about the networks of radicalisation woven into campus life. Several professors insisted to the Observer that some of the earlier killings had been about personal and professional disputes, even as they admitted that they had altered their own behaviour and routines to guard against becoming a target.

Signs calling for justice at the University Photograph: Emma Graham-Harrison/The Observer

It is a form of self-protection for more than 1,000 lecturers who face an almost unimaginable risk but have received little support from security forces beyond a warning to be careful.

Instead, they are watching their words even more closely than usual, and fear that fanatics who consider the education they offer to be too broad have made the first steps towards curtailing it.

“I have become much more self-conscious in our classes, being sure not to offend any groups, and all the time repressing myself, my own free will,” said a second colleague of Siddiquee’s, who asked not to be named. “I take care again and again not to identify myself with the views in class, saying that this is the author’s view, not mine.”

That creeping sense of oppression bothers most in the faculty even more than the threats to their lives. In response, they are demanding not greater security for academics, but greater police efforts to catch Siddiquee’s killers and dismantle the radical networks that fostered them.

The roads through the university have been painted with demands for justice in English and Bengali, a giant noose because students and colleagues alike want the perpetrators hanged, and clenched fists of those who have vowed to fight for Siddiquee.

The nervous university has banned his colleagues from setting up a platform in the gardens outside, which they hoped would serve as a memorial and focal point for weekly protests they plan to hold until his killers are found and brought to trial.

However, authorities fear this would set a precedent. Too many professors have been killed for all of them to be given a rallying point.