The Counter Terrorism and Transnational Crime (CTTC) unit has arrested a member of Ansar Al Islam for his alleged involvement in the murder of writer-blogger Avijit Roy.
The arrestee has been identified as Arafat Rahman alias Siam alias Sajjad, 24, an active member of operations department of the militant organization.
Dhaka Metropolitan Police's (DMP) Deputy Commissioner (Media) Masudur Rahman confirmed the matter on Saturday.
"Acting on a tip-off, a CTTC team arrested him from Aminbazar's Bordeshi area around 7pm on Friday," he said.
Arafat, in a primary interrogation, admitted that he and three of his cohorts hacked Avijit to death and critically injured his wife.
He claimed that the Ansar Al Islam team was led by Mukul Rana alias Sharif, who was killed in a gunfight with police's Detective Branch.
Arafat also admitted that they carried out more than one such killing operations under orders from sacked major Zia.
Police sources added that Arafat had confessed to being involved with the murders of Xulhaz Mannan, Niloy and Dipan.
The DMP had previously declared a bounty of Tk2 lakh for any information leading to the capture of Arafat.