Bangladesh's anti-terrorism unit has arrested what it called the prime suspect in the killing of Avijit Roy, a prominent Bangladeshi-American blogger who wrote against religious fundamentalism. He was hacked to death in Dhaka last week.
The Rapid Action Battalion arrested Farabi Shafiur Rahman in the capital Monday morning.
An RAB spokesman told the French new agency (AFP) that Farabi is a "fundamentalist blogger" who had threatened Roy through comments on Facebook and Twitter.
Bangladesh's The Daily Star reported that Farabi said on Facebook last year that it was "a holy duty of Bangalee Muslims" to kill Roy.
Roy, a Bangladesh-born U.S. citizen, and his wife, Rafida Ahmed, were attacked late Thursday after leaving a book fair at Dhaka University, authorities said. Ahmed, who is also a blogger, was seriously wounded and remains in the hospital.
The incident was the latest in a series of attacks on secular writers who have supported freethinking values in predominantly Muslim Bangladesh in recent years.
Farabi was arrested in 2013 after the murder of another blogger, but he was released on bail.
The Daily Star also reported there were witnesses to last week's attack, but none came forward to help the couple or apprehend the assailants.
An unknown militant group, Ansar Bangla 7, had earlier claimed responsibility for the attack. Roy "was the target because of his crime against Islam,'' the group tweeted. It was not immediately clear whether Farabi is associated with the group.
According to Roy's family, in recent weeks Islamist radicals had repeatedly threatened him because of his blog, "Mukto-mona," or "Freemind," that promoted humanist and rationalist ideas and condemned religious extremism.
Reporters Without Borders rated Bangladesh 146th among 180 countries in a ranking of press freedom last year.