The Central Bureau of Investigation (CBI) on Sunday wrote to all banks and asked them to report any irregularities in Letters of Undertaking, days after Punjab National Bank said LoUs worth thousands of crores were fraudulently issued to companies belonging to diamantaire Nirav Modi and his uncle, Mehul Choksi.

While the CBI is scrutinising the details of LoUs and letters of credit submitted by Punjab National Bank, in the past few days since the scam broke, PNB's head office too has scrutinised its LoUs and submitted a report to the CBI.

"(The) loss could have been higher than Rs 11,400 crore, since several LOUs were maturing in May 2018. These LoU(s) were for 180 days. (The) CBI is checking (the) amount involved in these LOUs," CBI sources told India Today.

Meanwhile, 10 suspended PNB officials were questioned in Mumbai on Sunday for around 8 hours and CBI's office by 7 pm.

CBI also questioned Nirav Modi's chief financial officer Vipul Ambani. He was called by CBI with documents. Vipul is the son of Dhirubhai's younger brother Natubhai Ambani.

On Saturday, CBI made the first arrest in a fast-widening investigation of one of India's biggest bank scams.

The CBI has detained three people - former PNB deputy manager Gokulnath Shetty, single window operator Manoj Kharat and Hemant Bhat, an authorised signatory of the Nirav Modi Group of Firms.

Gokulnath Shetty made explosive revelations: He admitted to having unauthorised access to a Level-5 password - the key for the SWIFT software that is used to issue Letters of Undertaking (LoUs).

He added that he shared the password with other individuals, essentially employees and directors of Nirav Modi's company.

An LoU is a guarantee from the bank that should the borrower default on repayment, it will pay back to the original lender.

The PNB issued the LoUs to firms linked with Nirav Modi and his uncle, Mehul Choksi.

The alleged scam began in 2011, and was detected in the third week of January this year, after which PNB officials reported it to the concerned agencies.

(With inputs from Mustafa Shaikh)

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