The girl's body was found on Wednesday in a paddy field.
The police had initially filed a murder case against an unidentified person but after the post-mortem report confirmed sexual assault, the charge of rape under IPC section 376 was added to the FIR.
According to additional SP (Barabanki) Ram Sewak Gautam, "The FIR, on Wednesday, was registered under section 302 of the IPC. On Thursday, we have added section 376 (rape) of the IPC based on the post-mortem report. We are questioning a few suspects after primary investigation."
According to the girl's father, she had gone to the fields in the day to harvest the crop.
"Around 5.30 pm, when she did not return home, I got worried and started looking for her. Around half an hour later, I went to the field and saw her slippers. A little ahead, I found her lifeless body," he said.
The victim was the eldest of four siblings. While her younger sister is around 15, her twin brothers are around seven.
However, there is confusion over the girl's age and the police have not added the charges under the Protection of Children from Sexual Offences (POCSO) Act.
While the family claimed that she was 17 years old, police said that in their first complaint, the family had said that the girl was above 18 years.
IG (Ayodhya Range) Sanjeev Gupta, meanwhile, said that the age of the victim would have no bearing on the investigation.
The victim was cremated on Thursday evening after the police assured them of arresting the accused at the earliest.