According to the girl's relatives, she was raped by Nirdesh (24) on January 12 night when she had gone out of her house and on January 13, she set herself ablaze.
The girl was admitted in a hospital after the incident, and succumbed to injuries on the same day, they said.
"Based on the complaint filed by the girl's father, an FIR under sections 376, 305 and 306 had been lodged on January 13," Deputy Superintendent of Police A C Srivastava said.
The post-mortem was performed on January 15.
The Dy SP said a probe had been ordered as to why the post-mortem was delayed when the body had been sent for the examination on January 14.
The post-mortem report established girl's death due to severe burn injuries. However, slides were sent for forensic examination to confirm the rape charges, Kheri SP Akhilesh Chaurasia said.