Bangladesh security forces have arrested a suspect in the killing of a Bangladeshi-American writer who was a prominent critic of religious extremism.

The arrest of Farabi Shafiur Rahman on Monday came four days after attackers wielding meat cleavers killed Avijit Roy as he walked with his wife on a crowded pavement in the capital Dhaka on February 26.

A spokesperson for the anti-crime Rapid Action Battalion said Rahman was arrested at a bus stop.

Mufti Mahmud Khan, the spokesperson, said Rahman had made death threats against Roy, adding that a case has been filed against Rahman, and that he would be handed over to the police. 

Mourners pay tribute to slain blogger in Dhaka

"The mystery will be revealed after the interrogation," Khan said.

Rahman, a Muslim blogger who denounced atheism, had threatened Roy in Facebook postings, Khan said, quoting him as writing: "Avijit Roy lives in America, so it's not possible to kill him right now. But he will be killed when he comes back."

Responding to his posts against "atheists", most of Rahman's followers said they ought to be killed to protect Islam.

While Rahman acknowledged making the threats, Khan said, authorities refused to say if they believed he was one of the attackers.

Roy, a Bangladesh-born engineer with American citizenship, was an outspoken atheist and critic of the intertwining of religion and politics.

He was killed while on a visit from Georgia in the US to attend Dhaka's main book fair.

He and his wife, Rafida Ahmed, were attacked after leaving the fair. Ahmed was seriously injured.

Rahman had been arrested previously for threatening an imam who performed funeral prayers for another atheist Bangladeshi blogger killed in 2013.

He was released on bail after six months in jail.

The Bangladesh government has accepted a US offer of FBI help in the investigation of the killing, according to AH Mahmood Ali, the foreign minister.

With reporting from Mahmud Hossain Opu from Dhaka.

Source: Al Jazeera and agencies