Machete-wielding assailants hacked to death a blogger in the Bangladeshi capital of Dhaka, in the latest of a series of attacks on writers who support freethinking values in the Muslim-majority nation.
The attack comes amid a crackdown on hardline Islamist groups, which have increased activities in recent years in the South Asian nation.
Avijit Roy, a U.S. citizen of Bangladeshi origin, and his wife and fellow blogger, Rafida Ahmed, were attacked on Thursday while returning from a book fair. Ahmed was seriously injured.
Police retrieved two machetes from the site, but have not yet identified any suspects.
They said they were investigating the involvement of Ansarullah Bangla Team, an Islamist extremist group based in Bangladesh that claimed responsibility on Friday for the killing.
Roy's family said Islamist radicals had been threatening him in recent weeks because he maintained a blog, "Mukto-mona," or "Freemind," that highlighted humanist and rationalist ideas and condemned religious extremism.
"Islamist radicals are behind my son's murder," Ajay Roy told reporters on Friday after filing a murder case with police.
"We mourn but we are not out," read a black banner on the site.
The Center for Inquiry, a U.S.-based nonprofit group Roy wrote for, said it was "shocked and heartbroken" by the killing.
"Dr. Roy was a true ally, a courageous and eloquent defender of reason, science, and free expression, in a country where those values have been under heavy attack," it said in a statement.
Media group Reporters Without Borders rated Bangladesh 146th among 180 countries in a ranking of press freedom last year.
In 2013, religious extremists targeted several secular bloggers who had demanded capital punishment for Islamist leaders convicted of war crimes during Bangladesh's war for independence.
Blogger Ahmed Rajib Haider was killed that year in a similar attack near his home in Dhaka after he led one such protest demanding capital punishment.
In 2004, Humayun Azad, a secular writer and professor at Dhaka University, was also attacked by militants while returning home from a Dhaka book fair. He later died in Germany while undergoing treatment.