Porabai, a student of Birra government school in Janjgir district, topped the list by scoring 484 marks out of 500. But the preliminary investigation conducted by the board suggested that it was "Munnabhai experiment" that got her highest marks in the board examination.

"The investigation revealed someone else wrote the answer sheets for the girl that was meticulously changed with the one she wrote in the examination hall," a senior board official said. The serial number of the answer sheets that the girl was given were different to the sheets that were evaluated, the official said.

The matter was exposed by CGBSE Chairman B K S Ray, who had some doubts over a village girl securing 484 marks in the board examination. "When the answer sheets of Porabai were taken out for verification, all the answers were perfectly written as if it were copied," Ray said. The irregularities became crystal clear when the Porabai's 10th board examination answer sheets were taken out as the writing also did not match.

Officials said that there were "enough evidences" to establish that "her original answer sheets" were replaced by second sets, written by someone else. According to Ray, Porabai was in fact an accused in the incident, but the involvement of education department officials and teachers could not be ruled out. The detail probe into the matter would bring the facts out, he said, adding that no one would be spared for the serious lapse committed to help the girl.

The board had cancelled the merit list of HSSC exam that was released with the result announced on Monday. Sources said that the board had decided to revaluate the answer sheets of the students who had secured place in top ten before releasing the revised merit list later.