The elderly couple who were found murdered in their Netaji Nagar home were strangulated and smothered to death, a preliminary post-mortem report has suggested.

Police suspect Dilip Mukherjee, 75, and Swapna, 65, were murdered between 10pm and 11pm.

Three labourers who were hired by the couple to paint the two-storey house, which they owned with relatives, have been detained.

A help of the Mukherjees had said a promoter had been calling up the couple with an offer to develop the corner plot.

Officers investigating the case said they were verifying the allegations made by the help and were talking to two promoters who had developed projects in the locality.

The sleuths are also examining the property documents to find out whether the childless couple had left any will and named anyone the heir to their property.

“We are probing the case from all possible angles,” said joint commissioner (crime) Murlidhar Sharma.

There were marks of violence on the body of the elderly woman, which suggest she had tried to resist the attacker.

“Her body was found at the landing of the staircase just behind the main door. There were several wounds on her body, suggesting she had tried to resist her assailants,” an officer of the homicide department said.

Mukherjee was found lying on the bed with his legs hanging.

“He was first gagged with a piece of cloth and then smothered with a pillow. He was pinned down on the bed,” the officer added.