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 amidst a crackdown on hardline Islamist groups, which have increased activities in recent years in the South Asian nation.
Avijit Roy, a US 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 murder.
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 US-based nonprofit group Roy wrote for, said it was "shocked and heartbroken" by the murder.