Police are examining eyewitness accounts, CCTV footage and recovered cartridges as BJP and TMC trade charges over post-poll violence in Bengal.
Sambhu Haldar, a resident of Ultadanga Main Road, and a neighbour of the victim, committed the heinous crime inside his room on the pretext of playing ludo.
80-year-old man apparently smothered to death, his 72-year-old wife strangulated using her own saree.
Sources said that the wife died of burn injuries, along with her seven-year-old son, Aakash Barman, who tried desperately to save her.
"Mondal is at large since the incident. We are looking for him. His wife and the rest of the family members will be interrogated," the official added.
The police zeroed in on small time businessman Shamsuddin after getting a lead from the sack which contained the chopped torso of the victim.
The police said that the couple strangulated their baby and threw the body in a pond near their relative's house on Sunday.
