As the CBI investigates a Class 11 student for the murder of Pradyuman Thakur at the Ryan International School in Gurgaon, the role of four policemen has emerged in the alleged fudging of evidence to incriminate a schoolbus conductor for the crime. CBI sources say their investigations have revealed "illegality and destruction of evidence" by the police in Gurugram, who had declared that seven-year-old Pradyuman was killed by bus conductor Ashok Kumar on September 8.