The slaying of a third secular blogger in as many months has set a chilling pattern in Bangladesh, a country long associated with a culture of moderation and free expression.
Machete-wielding assailants followed Ananta Bijoy Das on Tuesday morning when he left his house in the northeastern city of Sylhet and hacked him to death, police and friends said.
Das wrote for Mukto-Mona, the blog founded by Avijit Roy, a U.S. citizen who was killed in a similar attack outside a book fair in the Bangladeshi capital, Dhaka, in February. Another writer who protested Roy's killing on social media, Washiqur Rahman, was struck down on a Dhaka street in March.
Activists lay blame for the killings on ultraconservative Islamists who have gained prominence recently in the overwhelmingly Muslim country. They have complained about the slow pace of investigations and have accused authorities of allowing a culture of impunity to take hold.
Although successive governments have cracked down on hard-line Islamist groups, Bangladeshi politics have become intensely polarized, leading to what one regional expert described as a "governance vacuum."
The political turmoil appears to have emboldened Islamist extremists, including a new Al Qaeda affiliate that is claiming responsibility for at least two of the killings.
Who were the victims of these attacks?
Roy, 42, was a champion of atheism and outspoken critic of Islamists. A Bangladeshi, he moved to the U.S. in 2007, gained citizenship and worked as a software engineer in the Atlanta area. He wrote several books and founded the website Mukto-Mona, which means "free mind" in Bengali. Friends said his writings had made him the target of frequent death threats. But he returned regularly to his homeland to attend the annual book fair where he was killed Feb. 26.
Rahman, 26, worked for a travel agency and wrote satirical pieces about religion on Facebook and other sites. An admirer of Roy, he paid tribute to the slain blogger with a post on his Facebook page that read: "#iamavijit and words cannot be killed."
Das, a banker in his 30s, was a "relentless writer" on science and rationalism, according to an activist colleague, who requested anonymity to shield himself from reprisals. Das wrote the preface to a book written by Roy and also helped organize a new publication called Jukti, which means "logic," the friend said.
Have other writers been targeted?
There have been a number of attacks on secular academics and writers over the years. The recent deaths followed the 2013 killing of Ahmed Rajib Haider, a blogger who was hacked to death with machetes in Dhaka.
Who is responsible for their deaths?
Last week, Al Qaeda's affiliate in South Asia claimed responsibility for several killings of "blasphemers" in the region, including Roy and Haider.
A local militant group, Ansar al-Islam Bangladesh, said that Al Qaeda in the Indian Subcontinent had also claimed responsibility for the attack on Das, according to the SITE Intelligence Group, which monitors such groups online. But there was no immediate statement to that effect from Al Qaeda.
Police say they believe the attacks were carried out by homegrown militants who may have received training from the terrorist network. A local radical group, the Ansarullah Bangla Team, also claimed responsibility for Roy's death.
Al Qaeda's leader, Ayman Zawahiri, announced in September that the organization was expanding into the Indian subcontinent "to raise the flag of jihad" and bring Islamic rule to the region.
About 90% of Bangladesh's 160 million people are Muslim and its legal system is partially based on Islamic law, but it is governed as a parliamentary democracy.
Have there been any arrests?
Seven university students have been charged in Haider's killing, along with the leader of the Ansarullah Bangla Team, Jashimuddin Rahmani.
Police arrested two students who were overpowered by bystanders at the scene of Rahman's killing, but another suspect escaped.
A militant Islamist who allegedly threatened Roy on social media was also arrested but has not been charged.
Roy's family has criticized the pace of the investigation. His widow, Rafida Ahmed Bonya, who was seriously injured in the attack that killed Roy, this week accused the government of being "afraid of the extremists."
"Is Bangladesh going to be the next Pakistan or Afghanistan?" she asked in an interview with Voice of America.
Why has Bangladesh become a killing ground for bloggers?
The slain bloggers were targeted for their secular views, which Islamists said insulted their faith.
"The rise of this extreme form of violent Islam is occurring at a moment when Bangladeshi politics have become extremely polarized, and in a sense creating a governance vacuum," Alyssa Ayres, a senior fellow at the Council on Foreign Relations, told a House panel last month.
Opponents of Prime Minister Sheik Hasina Wajed have launched public strikes and engaged in street battles with security forces that have left more than 120 people dead since January.
The protests were timed to mark the first anniversary of a disputed election that brought Wajed and her Awami League party to power. A major opposition party, the Bangladesh Nationalist Party, boycotted the 2014 vote because it did not take place under a neutral caretaker government. Its leader, Khaleda Zia, a former prime minister, is now demanding that Wajed step down and that fresh elections be called.
The BNP is backed by the country's largest Islamist party, Jamaat-e-Islami, which has a student wing that is regularly accused of street violence, Ayres said.
Government forces have responded to the turmoil by arresting thousands of opposition activists and cracking down on the media.
Adding to the fractious environment is a tribunal seeking justice for atrocities committed in Bangladesh's 1971 war for independence from Pakistan. Activists said the four slain bloggers took part in or were supportive of a movement known as Shahbag, which sought the death penalty for Islamist leaders accused of war crimes.
Special correspondent Kader reported from Dhaka and Times staff writer Zavis from Los Angeles. Staff writer Shashank Bengali in Mumbai, India, contributed to this report.