The CBI on Thursday questioned four Reserve Bank of India (RBI) officials in connection with Rs 13,500 crore Punjab National Bank (PNB) fraud involving diamantaire Nirav Modi and his uncle Mehul Choksi, an official said.Those questioned includes three Chief General Managers and one General Manager of the central bank.Sources said the RBI officials were also questioned in connection with alleged benefits to diamond merchant Choksi's group of companies following 80:20 gold import scheme brought by former Union Finance Minister P. Chidambaram.Modi, a regular on the lists of rich and famous Indians since 2013, along with his group companies -- Diamond R US, Stellar Diamond and Solar Exports, uncle and business partner Choksi -- the promoter of Gitanjali Group, and others have been named in the Rs 13,500 crore scam which was admitted by the PNB in February, leading to a massive upheaval in the country's banking system.Modi left the country along with his family in the first week of January this year, weeks before the scam was reported to the Central Bureau of Investigation (CBI). His wife Ami, a US citizen, left on January 6 and uncle Choksi on January 4.The CBI in February had approached the Interpol with a request for issuing a Diffusion Notice against Modi to locate him. The PNB has claimed in different complaints to the CBI that several Letters of Undertaking (LoUs) were fraudulently issued by its officials in connivance with Modi and the other accused in the case causing huge losses to the banki. A LoU is a letter of undertaking issued by one bank to other banks, based on which foreign branches offer credit to buyers.