She allegedly committed the suicide last week but the matter came to the light after Childline India Foundation volunteers lodged an FIR with the police on April 25, about three days after the incident.
While her in-laws have claimed that the girl had hanged herself, the police registered it as a case of unnatural death as the body was recovered under mysterious circumstances.
Quoting the FIR filed at Sarangarh police station, its inspector in-charge Chandmani Singh said the girl was forced to marry on April 14 on the ocassion of Maha Bishuv Sankranti. The marriage was solemnised at the house of the bridegroom secretly.
Childline members had foiled her marriage in March this year and had cautioned her and her groom's parents that they would face consequences if she was married off before becoming a major.
Aloke Tripathy, co-ordinator of Childline in Kandhamal and Rasmita Karan, the district child protection officer blamed the police for not stopping the marriage of the girl.
"We had categorically brought the fact to the notice of the collector, the SP and the Child Marriage Protection Officer-cum-CDPO, K Nuagam who did nothing except lodging a station diary of the incident," he said.
Preliminary police investigation said the girl hanged herself in a suspicious manner. "It was only due to extreme mental strain and humiliations," it said.
A joint inquiry was conducted by the police, Childline, the district child protection officer and the child marriage protection officer, officials said.