A teenaged girl was murdered, allegedly by her elder sister, in her sleep at their Mohanpur Kachuara home late on Sunday after a heated argument.

Preliminary investigation hinted at honour killing but the truth emerged after family members were grilled for hours on Monday.

Alka Kumari, 17, was a Class XI student at a prominent school in Kankerbagh.

'We were informed about the incident around 10.30am on Monday by some residents. The girl was found dead in bed with her throat slit and veins of both wrists cut off. The police had a strong suspicion that all family members or at least one of them was involved in the murder. The girl's elder sister, identified as Priyanka (21), used to sleep in the same room and on the same cot. The police did not find any evidence of a forced entry into the house, further strengthening their hunch. Based on these facts, three of the family members - Alka's father Akhilesh Singh, who works for the central government, younger brother Saurabh (15) and elder sister Priyanka - were detained and questioned continuously,' Patna superintendent of police (east) Sudhir Porika said.

The police said Priyanka finally confessed to her crime during interrogation.

'Initially, the police suspected honour killing as the motive behind the murder. But Priyanka finally spoke up and admitted it was she who had murdered Alka. The family had fixed Priyanka's marriage but she did not want to get married right now. There was tension in the house because of the same and she was having heated arguments with her family members over the issue for quite some time. On Sunday night, when the two sisters went to sleep, Alka reasoned with Priyanka to listen to the family, angering Priyanka further. The girl was so disturbed by the nagging that she woke up after Alka had slept, took a knife and slit her throat. According to her, she did it in a fit of rage,' Porika said.

Another officer said Priyanka realised what she had done while Alka lay dying in bed.

'She came back to her senses and tried to make it look like a suicide. She cut the veins on Alka's wrist. Though the trio are still being questioned, it seems the family members knew, in the morning itself, that it was Priyanka who had killed Alka. They never informed the police and we got to know about the incident only from others. They might very well have been trying to hush up the case. More investigations are on,' another police officer at Gopalpur police station said.

A city-based psychiatrist said people can kill anyone in a fit of rage.

'It seems Priyanka was not in her senses when she carried out the crime,' said the psychiatrist on condition of anonymity.