A 13-year-old girl allegedly killed her younger sister with the help of her boyfriend and aunt and burnt the deceased's face with acid and chopped off fingers to conceal the identity in Bihar's Vaishali district, a senior police officer said on Thursday.

Police have arrested all three accused, including the minor, involved in the incident. While the 13-year-old girl has been sent to ‘Balika Sudhar Grah’ (correction home) in the district, her 18-year-old boyfriend and aunt are in judicial custody, an officer said.

Talking to reporters here on Thursday, Vaishali SP Ravi Ranjan Kumar, said the incident took place on May 15 at Harprasad village when the parents of the girls had gone to a relative's village to participate in a wedding ceremony.

When the parents returned home they found their younger nine-year-old-daughter missing, Kumar said, adding the parents lodged a complaint at Jandaha police station. The local police constituted a special investigation team (SIT) and lodged a missing report which was later converted into a murder case after the recovery of the body from a field behind her home on May 19, the SP said.

During investigation, police seized the mobile phones of the accused. "During interrogation, the girl and her boyfriend confessed to have committed the crime. The duo revealed to police that they killed the nine-year-old girl mainly because the deceased saw them in an awkward position …and they feared the younger one might reveal their love affairs to her parents," the SP said.

"After murdering the younger sister with a blunt object, the accused concealed her body in a box inside the home but after three days when the body began emitting a foul smell, they dumped it into a nearby field. They also burnt the deceased’s face with acid and chopped fingers to conceal her identity", said the SP.

Based on technical surveillance and scientific investigation, including analysis of their call detail records, police picked up the elder sister and her boyfriend and also cross-examined them.

Their interrogation revealed the mystery behind the incident. The girl’s 32-year old aunt was also arrested on the charge of helping the duo in the crime. Further investigation is on, the SP added.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.