The ayah who has been charged with the murder of 82-year-old Bijaya Das at her New Garia home last week had allegedly planned the crime before, police said.

As part of her plan, she left home that morning with her elderly mother, whom she dropped off at Garia station, with the promise to return soon.

The ayah had kept her promise and returned shortly after completing the “operation” at her workplace. Thereafter, the ayah, her mother, and her paramour fled to a location in South 24-Parganas on a local train, police sources said.

Ashalata Sardar and Mohammad Jalal Mir were arrested on Saturday morning for the murder of the elderly woman and a near-fatal attack on her husband, Prasanta Das.

Bijaya Das was found murdered under the staircase of her two-storeyed home at New Garia Cooperative Housing, with her hands tied up early on Friday. Her 87-year-old husband was found lying on the floor beside his bed in one of their bedrooms at their F-32 apartment.

Ashalata had started from her Narendrapur home early on the last day she reported to work at the Das residence, said the police. She had taken her mother along.

“She had dropped her mother at Garia station and asked her to wait. She, along with her paramour, Mohammad Jalal Mir, went to the New Garia home,” said an officer.

Ashalata entered the house, keeping the man waiting outside.

Ashalata, on her first visit to the house on August 17, had spotted the output monitor of the CCTV cameras installed in the house and noticed a blind spot near the staircase.

“She had called Bijaya to that exact spot and attacked her for her jewellery, thinking that her act would be outside the coverage area of the CCTV cameras,” said the officer.

By the time the crime was detected, Ashalata had allegedly disconnected the CCTV cameras and also switched off the power supply to the house.

However, the last footage retrieved from the cameras showed Ashalata inside the house and Mir waiting outside.

“The preliminary motive behind the murder appears to be theft. Ashalata had told that she had seen jewellery on the woman and had thought that they would help her set up a new home,” an officer said.

The police have not found any past criminal records of the arrested duo.