Al Amin Jonny, the prime suspect in the murders of a woman and her son in Kakrail, has admitted his involvement in the crimes.
Jonny, 33, has also said he acted alone, said Rapid Action Battalion (RAB).
RAB says the suspect, planned to kill Shamsunnahar, 46, after his sister Parvin Akter Mukta, 35, the third wife of Shamsunnahar's husband Abdul Karim, 56, tried to die by suicide following a quarrel with her husband.
RAB arrested Jonny in Gopalganj on Saturday morning while Karim and Mukta were being grilled in police custody in the case over the murders.
The other victim of the Nov 1 incident is Karim and Shamsunnahar's youngest son Sajjadul Karim Shaon, 18.
After initial interrogation of Jonny, RAB organised a media conference Saturday afternoon.
A bdnews24.com report said Shamsunnahar's brother Ashraf Ali named some unidentified people along with Karim, Mukta and Jonny in the case.
But RAB spokesperson Mufti Mahmud Khan said at the media conference that Jonny alone killed the woman and her son.