A 20-year-old commerce student has been arrested, along with two friends, for allegedly strangling his septuagenarian grandparents in their Sodepur house on February 16.
Partha Das, police said, had murdered Kalipada and Gita Das after being caught trying to steal from the house.
"Partha had been repeatedly demanding Rs 1 lakh from the couple to buy a motorcycle. The couple refused. The youth, along with Prabir Das, 18, and a 17-year-old boy, had gone to the house on February 16 in an attempt to steal the money," an officer of Ghola police station said.
"We have definite reasons to believe that Partha and his two friends were involved in the murder. They were picked up late on Sunday from their houses in Baguiati."
Kalipada, 75, a retired employee of Calcutta Port Trust, was found strangled with a gaamchha on his bed on the ground floor of his double-storeyed house at Indrapuri in Sodepur. His head was in a pool of blood that thickened over several hours.
In an adjoining room, Gita, 70, was found dead on her bed. She was strangled with a gaamchha, too, and her walking stick was lying on the floor. The bedcover was dishevelled, suggesting she had tried to resist the killer.
Kalipada had built the house around eight years back. The couple had another house a few metres away, in Sreenagar, whose tenant had fled a few months before the murder. They had a third house, in Lake Town, which they had sold a few years ago.
"The couple's only son Shyamal, a lawyer, had died in 2011. Soon after, daughter-in-law Manju moved to her mother's house in Baguiati with her children Sreya and Partha," a neighbour said.
"Manju would frequently visit the elderly couple. She had come a week before the murder to hire a cook for the in-laws. That was her last visit."
Within hours of news of the twin murders reaching her, Manju and her children turned up at the Sodepur house. Partha, sporting shorts and slippers, looked grim but unfazed.
He told Metro that he had visited his grandparents 48 hours before.
According to psychiatrist Jai Ranjan Ram, a criminal usually tries to gauge the response of others to his act and find out whether he is being suspected or not.
"If someone is actively involved in a crime, he or she will be curious to know how the incident was being viewed and the outcome of the investigation. He would hang around the crime scene and try to pre-empt certain situations," Ram said.
The police have learnt that on the evening of February 16, Partha had turned up at his grandparents' house with Prabir and the 17-year-old boy. He entered the house alone and then let in his friends behind the back of his grandparents, asking them to move to the first floor where the couple rarely went.
That night, Partha had dinner with his two friends on the first floor. Around 1.45am, while the couple were deep in sleep, Partha and his friends came to the ground floor and started looking for documents and money in the almirahs in Kalipada's room.
"The man woke up to the sound of the opening of an almirah. The trio pounced on him after he threatened to tell everyone that Partha had come to steal from his house. He died when they gagged him with a gaamchha," a police officer said.
The noise of the scuffle jolted Gita out of her sleep. Partha has apparently told the police that they had to kill her after she started screaming.
The police said Partha had once been arrested for theft.
"We kept tracking his calls and found that he was regularly conversing with these two friends. Other evidence suggested his involvement, too. He admitted to the crime when we were grilling him last night," the officer said.