In the wake of twin murders of old couples in the city, West Bengal Chief Minister Mamata Banerjee on Wednesday promised a thorough investigation in both cases and stringent action to ensure the safety of the elderly."Thorough investigation will be done of both the murders in Netajinagar and Narendrapur. I think that it is everyone's responsibility to see that no one is able to touch elderly couples' property," Banerjee told reporters.An elderly couple was found dead on Tuesday inside their house in the Netajinagar area here, amid allegations that they were under huge pressure from their maid servants and a promoter to sell off their property.Bodies of another married couple were recovered from inside trolley bags in the washroom of a garden house in West Bengal's South 24 Parganas district on the same day. They were employed as caretakers of the garden house since 1997.With regard to the Netajinagar incident, Banerjee said the place would be designated as a memorial of the couple so that no one was able to acquire the property."Only if we take some stringent action, these people will learn some lesson. Also, police will see who all are behind the murder of the caretakers in Kheyada (Narendrapur)," she said.According to Banerjee, in cases like these, the Panchayat members and municipality officers must work in tandem with the police as all of them work under the umbrella of the state.Police officers are working on various leads in both the cases.