Nirav Modi, the man at the centre of India's biggest banking fraud , has turned down CBI 's summons to return to India, telling the agency that he was too busy with his business abroad.The CBI said that Modi has not shared anything with regard to his current location. Modi has been accused, along with his uncle Mehul Choksi, of defrauding the Punjab National Bank of Rs 12,600 crore, with Rs 1,300 crore being added to the fraud kitty two days back.CBI had emailed the summons to Modi a few days ago, asking him to appear before it for questioning. Modi wrote back, “I have business to run here. Won’t be able to join.”CBI again wrote to him him, directing him to contact the Indian high commission in the country he was staying in so that he could be brought to India.The agency, according to some news reports, even offered to fly him back to India to join the probe.“It is mandatory for all the accused to join the probe,” CBI said and asked Modi to appear before it next week. CBI officials said they would have to trace him and use the extradition route to bring him to India. The agency may soon approach Interpol to issue a red corner notice against Modi and Mehul Choksi, sources said.Modi is at present facing a Blue Corner notice in connection with the scam. A Blue Corner notice -- to locate, identify or obtain information on a person of interest in a criminal investigation -- was issued by the Bureau of Immigration against the two, which will be valid for a year starting February 22 at all ports of embarkation.The CBI had filed the first FIR in the scam on February 14 against Modi, his wife Ami, brother Nishal, Choksi and his firms Diamond R US, Solar Exports and Stellar Diamond. The agency stated that the fraud involved an amount of around Rs 6,400 crore. Modi, his family and Choksi left the country in early January.The CBI filed a second FIR on February 15 for Rs 4,886.72-crore fraud against Choksi's Gitanjali Group. To date, 12 persons have been arrested by the CBI in connection with the case. The Enforcement Directorate has claimed to have attached properties worth Rs 6,000 crore.