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DHAKA, Bangladesh — Machete-wielding assailants hacked to death an American blogger, the latest of a series of attacks on writers who support free-thinking values in Muslim-majority Bangladesh.

The attack comes amidst a crackdown on hardline Islamist groups, which have increased activities in recent years in the South Asian nation.

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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 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.

The Center for Inquiry, a U.S.-based nonprofit group Roy wrote for, said it was "shocked and heartbroken" by the murder. "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.

Avijit Roy's wife Rafida Ahmed Banna is carried on a stretcher to Dhaka Medical Collage on Friday after being seriously injured. RAJIB DHAR / AFP - Getty Images

— Reuters