Bangladesh police said on Saturday they had arrested an Islamist militant wanted in connection with the 2015 killing of a US blogger critical of religious extremism.
Deputy police commissioner Masudur Rahman said the man, identified as Arafat Rahman, 24, a member of al Qaeda-inspired militant group Ansar Ullah Bangla Team, was suspected of taking part in the killing of writer Avijit Roy.
Roy, a US citizen of Bangladeshi origin, was hacked to death by machete-wielding assailants in February 2015 while returning home with his wife from a Dhaka book fair. Roy's widow, Rafida Ahmed, was seriously injured.
Police official Rahman said the detainee, who was identified after analysing CCTV footage, was arrested by the counter-terrorism police unit on the outskirts of the capital, Dhaka, on Friday night.
"In the primary interrogation, he confessed his involvement in the killing of four other secular activists," he told media.
It was not possible to contact the detainee to comment as he was in police custody.
Muslim-majority Bangladesh of 160 million people has had a string of deadly attacks targeting bloggers, foreigners and religious minorities.
The most serious recent attack came in July 2016, when gunmen stormed a cafe in the diplomatic quarter of Dhaka and killed 22 people, most of them foreigners.
Police say the Ansar Ullah Bangla Team militant group is behind the murders of more than a dozen secular bloggers and gay rights activists.
They believe a sacked army major, who is still at large, was the leader of the group and masterminded the killings.
Al Qaeda and Islamic State have also claimed responsibility for a series of killings over the past few years, including that of Roy.
The government has denied the presence of such groups, blaming domestic militants instead. But security experts say the scale and sophistication of the cafe attack suggested links to a wider network.
Police and army commandos have killed more than 60 suspected militants and arrested hundreds since the cafe attack.
