The Central Bureau of Investigations (CBI) raided the offices of a well-known legal firm as part of the ongoing probe into the Rs 11,300 crore Punjab National Bank (PNB) fraud involving billionaire diamond trader Nirav Modi, official sources said here on Monday.The CBI sleuths last week swooped on the offices of Cyril Amarchand Mangaldas, a leading firm which was reportedly hired by Modi around a month before the scam broke in mid-February, at their offices in Lower Parel here.The CBI examined the documents provided by the firm pertaining to the Nirav Modi-related mandate, which the company later reportedly terminated after the scam erupted on February 14.Business and political circles are viewing the raid in the light of Prime Minister Narendra Modi's assertion that his government will "take stern action" in any economic irregularity.Addressing a global leaders summit on Saturday, Modi had said: "The system will not accept the theft of public money. That is the key to New Economy, New Rules."CBI Director Alok Kumar Verma too on Monday directed the officials in charge of the investigation to "complete (it) within the stipulated time" and has also asked them "not to spare anyone found involved".Political sources pointed out that Cyril Shroff of Cyril Amarchand Mangaldas has family ties through marriage with noted industrialist Gautam Adani. The questioning of Vipul Ambani, of the Ambani family, is also being viewed in the same light, they added.Despite repeated attempts, senior officials at the Mumbai branch of Cyril Amarchand Mangaldas -- which has offices pan-India -- were not available for comments in the matter.