The alleged incident of gangrape had taken place on July 4 when the woman was alone at her residence at Sandeshkhali in North 24 Parganas district of the state, they said.
She died in a state-run hospital here yesterday, the police said.
"The miscreants had inserted an iron rod and a broken bottle into her private parts to kill her after the gangrape. They had thrown her outside her house into the bushes assuming her to be dead," said a senior police officer quoting the woman's statement to the police.
"Her injuries were grave. Her private parts were brutally damaged by the broken bottle stuck inside. Doctors had a tough time in conducting operations. She suffered too much pain and finally succumbed late last night," a senior doctor of the hospital said.
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The woman used to run a roadside food stall near Sandeshkhali police station, her elder son, who lives elsewhere, said, adding she had returned home at around 10 PM.
Her younger son, who lives in the same house, had gone to his in-law's place and the woman was sitting in the balcony past midnight when the accused along with four others caught hold of her from the back, the police officer said.
A case was registered in the matter on July 5 and the prime accused, who was named by the victim, was arrested on July 6 and sent to judicial custody, the police told PTI.
West Bengal additional director general of police (law and order) Anuj Sharma said, "Police are looking into the case very seriously. Strict action will be taken against those who are guilty."
State Women Commission Chairperson Leena Ganguly said the panel would file a suo motu case if the victim's family lodge a complain of police negligence in conducting probe into the incident.