A list of eighty-four bloggers was sent to newspapers in 2013. Four are now dead. Illustration by John Hersey

On the afternoon of February 26th, Avijit Roy was in Dhaka, finishing a column for BDNews24, a Bangladeshi Web site of news and commentary. Its title, in Bengali, was “Why Is There Something Rather Than Nothing?,” and it adapted ideas from his new book, a primer on cosmology. For Roy, who was forty-two, science trumped religion. He took after his father, Ajoy, an emeritus physics professor at Dhaka University and an ardent rationalist. “I don’t bother about whether God exists,” Ajoy Roy told me. “Let him do his business, and let me do my business.” Avijit, even more vocal than his father, liked to compare faith to a virus—infecting human beings and impelling them into conflict. He once wrote, “The vaccine against religion is to build up a scientific approach.”

Roy and his wife, Rafida Ahmed, an executive at a credit-rating agency, lived in Atlanta. They had fallen in love from afar: in 2001, Roy started a collective blog called Mukto Mona, or Free Thinker, and Ahmed wrote to him after reading one of his posts, agreeing with his dismissal of religion as “fairy tales.” In 2006, Roy moved to Atlanta, where he worked as a software architect. But his real interests emerged in his blog posts, and in several books in which he dismantled the dogmas of religious belief—of his own Hindu background, but also of Islam, the state religion in Sunni-majority Bangladesh. “He was an addabaaj,” his father said. He used the word to mean “gossip,” but it also hinted at his son’s love of argument.

Mukto Mona’s comments section often drew irate Islamists, and Roy waded into earnest debates with them. He could seem as inflexible as the people he bickered with, refusing to acknowledge any grace or meaning that religion might grant its faithful. When one commenter claimed that the Koran was a repository of scientific wisdom, Roy asked why the Islamic world was “so behind in science and technology?,” and added, “Even Israel has more scientists than all the Muslim countries nowadays.” His father warned him that he was “too passionate.” On Facebook, one extremist wrote, “Avijit Roy lives in America, so it’s not possible to kill him right now. But he will be killed when he comes back.”

When Roy told his parents that he planned to visit in February, his father tried to dissuade him. “Dhaka is now not a very good place. The law-and-order situation is worsening day by day,” Ajoy Roy said. “I pointed out, ‘You’re a targeted person. Your name has been publicized as an atheist.’ ”

Roy and Ahmed went anyway, staying at her family’s house, not far from the city center. After finishing his column, Roy wanted to visit the Ekushey Book Fair, where hundreds of booksellers and publishers gather every February to celebrate Bengali literature. Ahmed and Roy attended an event hosted by Roy’s publisher before browsing through a section of children’s books. A photograph on Facebook shows them sitting on the ground. Roy, wearing a red kurta, is looking down; next to him, Ahmed reaches into a paper bag for a snack.

At around 8 P.M., as they walked toward their rented car, a young boy asked Roy for a handout. He gave the boy a hundred takas—a little more than a dollar—and an admonition to go home. Ahmed doesn’t recall the men who rushed at Roy and hacked at him with machetes, and she doesn’t recall trying to stop them. She received several wounds to her head and another that severed her left thumb. Later, in photographs of the attack, she noticed that there had been policemen standing nearby; they did nothing to intervene. Roy fell to the sidewalk, face down; his attackers dropped their weapons and ran away. By the time his father reached the Dhaka Medical College Hospital, Roy was dead.

Roy’s murder was claimed by a Twitter account belonging to the Ansarullah Bangla Team, an Islamic militant group. He was an American citizen, the tweets noted, and his death avenged the actions of the United States against ISIS. A Bangladeshi police official called the group the “closest relative” of Al Qaeda on the Indian subcontinent, and it has been linked to the murders of at least five other secular voices—the first in 2013, but the others since Roy’s death, at the rate of roughly one every other month. In October, when I visited Dhaka, there had been no attacks for eleven weeks, and the writers I met seemed to be steeling themselves for bad news. Three days after I left, Roy’s publisher was killed in his office, and, elsewhere in the city, another publisher and two bloggers were attacked.

Of the six who have died, four were on a list of eighty-four “atheist bloggers,” which was sent anonymously to newspapers in 2013. In nearly every attack, the weapon has been a machete. Two dozen suspects have been arrested, but so much doubt persists over the killings—and over the government’s handling of them—that Dhaka is rife with conspiracy theories. Some of the bloggers who number among the eighty-four revealed suspicions that the state’s security agencies ordered the hits.

Ahmed and Roy hadn’t anticipated Bangladesh’s lurch into murderous extremism. “I don’t think we missed it because we were away,” Ahmed told me. “I think this is a sudden shift, but it has been cooking for a while.” A few days before his death, Ahmed said, Roy had given her a tour of the places where he grew up. “We walked around the university campus. He showed me where he lived when he was little. He showed me his elementary school. He used to say, ‘Who will touch me in my own neighborhood?’ ”

In Dhaka, conversations about the killings inevitably circle back to 1971, when Bangladesh broke away from Pakistan, whose strict Islamic pieties and Urdu culture encroached on Bengali liberalism. The Ekushey Book Fair occupies a sprawling park called Suhrawardy Udyan, where, in March, 1971, a politician named Mujibur Rahman urged an audience of two million to embrace civil disobedience and turn East Pakistan into an independent Bangladesh. The speech, an electric moment in Bangladesh’s history, is depicted in posters that still hang in many living rooms in Dhaka.

The ensuing “liberation war,” as Bangladeshis call it, is commemorated in a museum in the park, a half-buried, brutalist gallery whose raw-concrete shell staves off Dhaka’s soggy heat. Photographs of corpses, alone or in great piles, often charred, run along one wall. Some estimates suggest that Pakistan’s armed forces killed half a million people in the nine-month war, but most Bangladeshis—in particular, those from the Awami League, the political party that Rahman once led—say that the toll was closer to three million; they also call it a genocide. Early in December, 1971, the Indian Army intervened, hastening Pakistan’s defeat. Two weeks later, in Suhrawardy Udyan, the commander of Pakistan’s occupying forces surrendered, granting Bangladesh its independence. The war’s violence and the actions of Bengalis who collaborated with Pakistani forces remain the source of many of Bangladesh’s political questions. The word razakar, or “volunteer,” once used to describe members of pro-Pakistan militias, has entered colloquial Bengali as a scathing pejorative.