Police on Monday recovered two knives that had allegedly been used to slit the throats of four persons in a Behala house last week from a canal in Bishnupur, South 24-Parganas.
A screwdriver with which one of the victims had been hit on the head was found in a pond, 10km from the canal.
Four persons had been arrested in connection with the murders on Sunday.
The bodies of cable operator Dipak Bhattacharya, 52, and his mother Gouri, 73, were found in their respective rooms on the first floor of the two-storey house.
The bodies of their help Anima Mondal and ayah Moyna Roy were found on the ground floor.
On Monday, the accused Munna Dhali, Rajesh Das, Naba Dhali and Sattar Mondal were taken to the spot where they said they had dumped the weapons, said Pallab Kanti Ghosh, joint commissioner of police (crime).
According to investigators, the four held wanted to grab the money collected from those who subscribe to Bhattacharyas cable service. While the knives were new, the screwdriver allegedly belonged to Munna, who was employed by the Bhattacharyas.
Munna used the screwdriver for emergency repairs at subscribers houses. One of the assassins had stabbed Moyna with the tool, said an officer.
Ghosh said: We are yet to identify the persons who actually slit the throats of the victims.
The investigators plan to take the accused to the Behala house to reconstruct the events on the day of the murder.