NEW DELHI, March 6 (Xinhua) -- India's biggest-ever bank fraud of 1.8 billion U.S. dollars is getting murkier with each passing day.

After the state-owned Punjab National Bank, which is the epicentre of the fraud, two of the country's leading private sector banks -- ICICI Bank and Axis Bank -- are now under the scanner.

"India's Serious Fraud Investigation Office has summoned ICICI Bank's managing director Chanda Kochhar and Axis Bank's head Shikha Sharma in connection with the fraud orchestrated by billionaire jeweller Nirav Modi and his uncle Mehul Choksi," sources said Tuesday.

ICICI Bank is said to have led a consortium of 31 banks that lent money to the Gitanjali Group, a jewellery company owned by Choksi, according to sources.

Indian investigators have so far arrested nearly 20 people, including some senior executives of companies owned by Modi and Choksi as well as high-ranking officials of the Punjab National Bank, in the bank fraud probe.

Though Modi has not yet been arrested, India's External Affairs Ministry has revoked his passport as well as Choksi's for their involvement in the scam.

Indian authorities have also launched a massive crackdown on companies linked to Modi, and seized 10,000 luxury watches and nine luxury cars last week, apart from freezing his personal shares and mutual funds.

Modi is said to have defrauded Punjab National Bank, India's second largest state-run bank, of 1.8 billion U.S. dollars, though he said that he only owed the bank 775 million U.S. dollars, in a letter sent to the bank's management.

The celebrity jeweller -- the mastermind of the massive fraud -- is said to have fled the country and was reportedly last seen in New York after his appearance at the World Economic Forum in Davos as a member of the Indian delegation.