A leading Mumbai law firm may face action by the Central Bureau of Investigation (CBI) in connection with the possession of some unauthorised documents pertaining to the ongoing probe in the Punjab National Bank (PNB) fraud case, official sources said here on Monday.When contacted by IANS, an official spokesperson from the law firm said: "We have no comments to make in the matter."According to CBI sources, the firm in question was found holding certain documents belonging to PNB which were handed over to absconder diamantaire Mehul Choksi by the arrested bank officer Gokulnath Shetty.Since it was risky to keep the incriminating documents in the bank, Shetty had passed them on to Choksi, who in turn gave them to the law firm for safe-keeping, the sources said.The documents were collected during a "raid" in February by the CBI at the law firm, which was hired by accused diamond merchant Nirav Modi before the PNB admitted to the mega-scam on February 14.The CBI revealed that it had learnt of unauthorised movements of sensitive and scam-related documents from the bank to other locations -- and then to the law firm.