The body was found hanging inside the factory in Kiwana village under the Kandhla police station limits on Tuesday, Station House Officer (SHO) Rozant Tyagi said.
The accused have been arrested and the victim rushed to the district hospital where his condition was stated to be serious.
The girl has been rushed to Kanpur for specialised treatment as she has received severe burn injuries, police said.
Under the NSA, one can be detained without a charge for up to 12 months if the authorities are satisfied that the person is a threat to national security or law and order.
There were multiple injuries on the body indicating it was a murder,” said Shahjahanpur SP S Anand, adding that some of the classmates of the victim were taken into custody.
The girl was rescued from his house on Saturday morning, Lalauli police station SHO Sandeep Kumar Tiwari said.
The mother said the accused left her daughter in the field in an unconscious state after committing the rape and fled the place.
Kusum got seriously injured after her husband attacked her with an iron rod when she tried to prevent him from drinking.
She was married to Afsar for over an year and was allegedly being harassed by her in-laws for not bringing enough dowry with her, they said.
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Referring to the police report lodged on the basis of the woman's complaint, SP Kumar said after the accused, his younger brother also raped the woman who was 12 at that time.
The girl was allegedly taken to a nearby forest area by force by a Class 10 student of the same school and was raped.
The victim said she was forced to stay silent after being threatened that the video would be put out on social media.
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The police claimed that the accused had admitted that he lured the girls to the fields with a packet of biscuits and tried to rape them.
When their son came home on Thursday morning and enquired about his mother, the accused told him about the incident and fled, police said.