He issued a confessional statement to the court of Judicial Magistrate KM Mohiuddin on Wednesday, police and lawyers say.
Seven men, including former city councillor Nazrul Islam and lawyer Chandan Sarkar, were abducted on Apr 27. Their bodies were found floating in the Shitalakhyya River several days later.
Nazrul's father-in-law Shahidul Islam had claimed earlier that RAB took Tk 60 million from prime suspect Nur Hossain for the crime.
Three former officers of the elite police unit -- ex-Lt Col Tarek Sayeed Mohammad, then RAB-11 commander, Maj Arif Hossain and Lt Commander MM Rana -- were removed from RAB and later sent on forced retirement.
They were arrested in line with a High Court order and are currently in police custody.
Arif Hossain's confessional statement comes after police questioned him for 18 days.
Court officials say he gave a 20-page statement each in the murder cases of Nazrul and Chandan.
It was not clear what the former RAB officer said in his statement. But Narayanganj's Additional Public Prosecutor Fazlur Rahman told reporters that they had learnt that Arif described the crime.
"We learnt that Maj Arif Hossain, in his statement, had given a vivid description of the incident describing how the men were abducted and where they were killed," he claimed.
The state counsel added the case's prime suspect Nur Hossain's complicity with the incident was also highlighted.
However, former RAB-11 commander Tarek Sayeed, son-in-law of minister Mofazzal Hossain Chowdhury Maya, has been denying the charges labelled against him.
Witnesses have told a judicial investigation committee that they saw several men dressed in RAB uniform during the abduction.
During Wednesday's hearing, court police's Sub-Inspector Mohammad Ashraf said the accused had picked up five men, including Nazrul, on Apr 27 from the Dhaka-Narayanganj Link Road.
"RAB personnel abducted Chandan Sarkar and his car driver as they were recording the incident on mobile," he added.
Police sought another seven days to question the other two suspects, Tarek Sayeed and MM Rana, but the court granted five instead.
Arif has been sent to police custody. He has been questioned three times at the district police lines since his arrest on May 11.
Chief of Narayanganj District Lawyers Association Sakhawat Hossain told reporters that their allegations had been proven with Arif's confession.
"The former officers had affinity with Nur Hossain. He has perpetrated the murders bribing these three RAB officers," he claimed.
