Punjab National Bank (PNB), facing an almost $2-billion fraud, has appointed consultancy firm BDO to undertake forensic audit of five group companies of jeweller Nirav Modi, the key accused in the scandal. \u201cIt shall be our endeavour to exercise every bit of diligence in executing this assignment, given its sheer scale and complexity,\u201d Kartik Radia, BDO India partner and head (risk and advisory services), said in a statement. Separately, in a filing to stock exchanges on Thursday, PNB said it appointed the Belgium-headquartered audit firm on Tuesday. Earlier this month, PNB said letters of undertakings worth $1.77 billion were issued fraudulently by one of its branches in Mumbai to firms of jewellers Nirav Modi and Mehul Choksi through the SWIFT interbank system, without making corresponding entries in its core banking solution, as was the procedure. Since both SWIFT and CBS were not linked, such activities escaped tighter scrutiny, causing the massive fraud, it had claimed.