The investigation has revealed illegality and destruction of evidence by Gurugram Police in the Pradyuman murder case, CBI sources said on Sunday.
Sources said the Gurugram Police, which was probing the case initially, had destroyed the evidence and made a "false' arrest" when it claimed Ashok was the "killer."
Whether the evidence was destroyed purposely and a knife was 'planted' on Ashok deliberately or it was poor policing, it is still being investigated, sources added.
Four policemen of Gurugram Police (SIT) are under the scanner and their call detail records are being analysed. A departmental enquiry has been recommended against them, sources said.
They further added once the CBI took the case from Gurugram Police, it had started probe the role of investigating officers.