Dhaka (AFP) - Bangladesh authorities announced they had arrested a radical Islamist Monday over last week's machete murder of an atheist American blogger, accusing him of being behind earlier death threats on social media.

A spokesman for the elite Rapid Action Battalion (RAB) said Farabi Shafiur Rahman had been arrested at a bus station in the capital Dhaka over the gruesome killing of Avijit Roy, describing him as the prime suspect in a crime that triggered international outrage.

"He is the main suspect," RAB spokesman Major Maksudul Alam told AFP.

"He is a fundamentalist blogger," the spokesman said, adding that according to "primary information" Farabi had threatened Roy through Twitter and Facebook.

"(Roy's) family told us that he got threats from Farabi several times," he added.

The American, who earned fame for his blog pieces and for authoring a series of books including the best-selling "The Virus of the Faith", was hacked to death with a machete in downtown Dhaka last Thursday.

Roy was the founder of the Mukto-Mona (Free-mind) blog. He was born in Bangladesh to a family of scholars but moved to Atlanta in the southern US state of Georgia around 15 years ago.

He and his blogger wife, who was also badly injured in the attack, had been returning from a book fair when they were both hauled off their rickshaw by two assailants who then slashed them with the machetes.

The RAB paraded Farabi before the press at its headquarters in Dhaka where another RAB spokesman, Mufti Mahmud, described him as a member of the banned pan-Islamist outfit Hizbut Tahrir.





- 'He'll be murdered' -





"On different occasions, he exchanged (Roy's) location, his identity and his family's photographs with various people," Mahmud told reporters.

"He wrote: 'Avijit Roy lives in America. So it's not possible to kill him at this moment. But when he'll return to the country, he'll be murdered'," Mahmud said.

A source within the RAB, which is mainly tasked with tackling religious militancy in the Muslim-majority nation, said correspondence between Farabi and another person about killing Roy had been discovered.

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A former physics student at a top university, Farabi was detained in 2010 after he joined Hizbut Tahrir and was arrested again after the murder of another atheist blogger Ahmed Rajib Haider in February 2013, but was released on bail.

Roy was the second atheist blogger to have been murdered in Bangladesh in the last two years and the fourth writer to have been attacked since 2004.

His killing was greeted by uproar both at home and abroad with hundreds of secular activists holding protests for days to demand the immediate arrest of the perpetrators.

They also slammed the country's secular government for not doing enough to protect humanist writers.

The United States condemned the killing as a "shocking act of violence" and an assault on the country's "proud tradition" of free speech.

"This was not just an attack against a person, but a cowardly assault on the universal principles enshrined in Bangladesh's constitution," State Department spokeswoman Jen Psaki told reporters.

Roy's father, Ajoy, said he had warned his son he could face problems in Muslim-majority Bangladesh, where secular activists have expressed concern about the rise of Islamism.

On Sunday, Ajoy Roy reiterated that Islamist militants were responsible for his son's death, but also blamed the police for failing to protect him despite repeated threats on his life.

"I am speechless at this moment of mourning. When the fundamentalists threatened, I informed the Inspector General of Police and Deputy Inspector General of Police," Ajoy Roy said, referring to the national police chiefs.

"This murder has proved their utter failure."