They said the examination is focussed on understanding how the crime was detected by the bank and other procedural issues and their violations.

They are not being treated as accused, the officials said.

It has been alleged that fraudulent guarantees were issued to the uncle-nephew duo of Nirav Modi and Mehul Choksi through 293 Letters of Undertakings.

Refusing to give out any further details, the officials said the agency is also questioning the 12-accused arrested by the agency so far.

They said this is the first time the agency has examined the top brass of the Punjab National Bank in connection with the fraud, which remained undetected state for over six years, shaking up the banking system of the country.

According to the website of the PNB, Rao is in-charge of the integrated risk management division, credit (operations), monitoring of accounts with exposure of above Rs 50 crores, including stock audit and Portfolio Management Service (PMS), industrial rehabilitation and action taken on borrowal frauds reported from Mumbai, where the alleged fraud took place.

Rao started his banking career in Vijaya Bank in the year 1982 as a Probationary Officer, it said.

Mehta started his banking career as an agriculture field officer at the Allahabad Bank in 1982.

The CBI has so far arrested 12 accused in the case.

Vipul Ambani, president (finance) of Fire Star Diamond, executive assistant Kavita Mankikar and senior executive Arjun Patil (all from Nirav Modi's companies) were arrested on Tuesday.

Kapil Khandelwal, CFO of Nakshatra group and Gitanjali group, and Niten Shahi, manager of Gitanjali group, were also arrested by the agency the same day. An authorised signatory of Nirav Modi's company was also arrested.

From the PNB, the agency had arrested Rajesh Jindal, a general manager, retired official Gokulnath Shetty and Single Window Operator Manoj Kharat, besides three other officials -- Bechhu Tiwari, Yashwant Joshi and Praful Sawant.

Shetty and Kharat allegedly sent messages of the LoUs using an international messaging system for banking systems called SWIFT (Society for Worldwide Interbank Financial Telecommunication), which is used to pass instructions among banks globally to transfer funds, but did not make entries in the bank's system to avoid detection.

Based on these instructions and guarantees, funds were released by overseas branches of Indian banks.

It is alleged that Rs 11,400 crores of LoUs were fraudulently issued to the firms of Nirav Modi and Choksi.