Thakur Das, 62, a retired engineer of the Damodar Valley Corporation (DVC), was found dead in the bedroom of his partly-constructed, two-storeyed house in New Barrackpore on Thursday.

The body was discovered by Das? maid, when she came in for work in the morning. In a statement to police, the part-time domestic help said she had found the main door open and Das sitting in a chair.

?I was surprised to see him sitting like that so early in the morning. I called him a few times. When he did not respond, I went near and shook him a little. He slumped to the floor. Only then did I realise that babu was dead,? the statement reads.

Das? wife Maya was away, visiting their only daughter in Tiljala. Alerted by the maid?s screams, neighbours rushed to the Das household and later called the New Barrackpore police outpost.

Police first detained and later arrested three masons living on the partly-finished first floor. Das had engaged them to construct the floor for his son, a private bank employee posted in Behrampore.

Das, who was apparently strict about letting callers into the house, was friendly with the masons, even attending the marriage of one in Murshidabad.

Investigation revealed that Das ?might have been strangulated to death? and then made to sit on the chair. ?The killers were tipped off in advance about the absence of family members,? said Parveen Kumar, superintendent of police (North 24-Parganas).

?They had come in a car around 2.30 am on Thursday and entered the house without breaking open the door. That the door was not locked from inside forces us to think about the role of an insider in the murder. The arrested masons confessed that Das owed a large amount to them and they were putting pressure on him to pay up,? Kumar explained.

?The almirahs and other places where the family kept its valuables were thoroughly searched by the miscreants, but they did not know that Das kept all his valuables in the bank,? Kumar added.

Police found Das? friendship with the masons suspicious. ?He did not allow his son?s friends to enter the house, but allowed the masons to stay. Also, while the miscreants were ransacking the ground floor, the masons failed to raise an alarm,? an officer observed.