All the three accused, including a minor girl, involved in the incident have been arrested.

A 13-year-old girl allegedly killed her younger sister with the help of her boyfriend and aunt and burnt her face with acid and chopped off her fingers to hide her identity in Bihar's Vaishali district, police said on Sunday. This information has been given by a senior police officer.

According to the police officer, all the three accused, including the minor girl involved in the incident, have been arrested. The 13-year-old girl has been sent to a 'girls' correction home' in the district, while her 18-year-old boyfriend and aunt are in judicial custody.

Vaishali Superintendent of Police (SP) Ravi Ranjan Kumar told reporters on Thursday that the incident took place on May 15 in Harprasad village when the girls' parents had gone to a relative's village to attend a wedding.

When the parents returned home, they found their younger nine-year-old daughter missing. Following this, the girls' parents lodged a complaint at Jandaha police station, the SP said. The SP said the local police had formed a Special Investigation Team (SIT) and registered a missing person's report, which was later converted into a murder case after the body was found in a field behind his house on May 19.

During the investigation, the police seized the mobile phones of the accused. "During interrogation, the girl and her boyfriend confessed to committing the crime... The duo told the police that they killed the nine-year-old girl because she had seen them in an objectionable position. And they were afraid that the younger sister would expose their relationship to their parents..."

According to the SP, "After killing the little girl with a blunt object, the accused hid her body in a box inside the house, but three days later when the body started stinking, they threw her in a nearby field... They also burnt the little girl's face with acid and chopped off her fingers to hide her identity..."

On the basis of technical surveillance and scientific investigation, including the investigation of phone call details, the police caught the elder sister and her lover and questioned them, which revealed the secret of the incident. The girl's 32-year-old aunt was also arrested for helping the duo in the murder. The investigation is still going on.