NEW DELHI, March 7 (Xinhua) -- Indian investigators Wednesday grilled the top boss of the state-owned Punjab National Bank, which is epicenter of the country's biggest-ever bank fraud of 1.8 billion U.S. dollars orchestrated by billionaire jeweller Nirav Modi.

"Punjab National Bank's CEO Sunil Mehta was summoned by India's Serious Fraud Investigation Office in Mumbai and investigators questioned him and recorded his statement before letting him go," sources said.

The heads of India's two leading private banks - ICICI Bank and Axis Bank - are next in line to be questioned by the Serious Fraud Investigation Office, which Tuesday issued summons to them to appear before it in the scam that has rocked the country.

Indian investigators have so far arrested nearly 20 people, including some senior executives of companies owned by Modi and his uncle Mehul 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 has said that he owed the bank only 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 World Economic Forum in Davos as a member of the Indian delegation.