Investigating officer Mamunur Rashid Mandal said detectives arrested Sepoy Asaduzzaman past midnight on Sunday over the murders in April.
The latest arrest takes the number of people held in the case to 30 -- 17 of them RAB personnel.
Police produced him before the court of Narayanganj's Senior Judicial Magistrate KM Mahiuddin on Sunday and sought a 10-day remand. The court granted seven days.
Seven, including Narayanganj City Councillor Nazrul Islam and senior lawyer Chandan Kumar Sarkar, were abducted from the city on Apr 27. Their bodies were found floating on the Shitalakkhya River a few days later.
Former Lt Col Tarek Sayeed, then the RAB-11 commander, was removed along with former Maj Arif Hossain and former Lt Commander MM Rana even before the bodies were found.
The former RAB trio was forced to retire and arrested.