(Updates with details, background; adds dateline)
MUMBAI, Feb 27 (Reuters) - India's Punjab National Bank (PNB) said the amount of fraudulent transactions involving the bank could go up by about $204 million in addition to the $1.77 billion it had earlier reported, sending shares down as much as 8.9 percent to a 20-month low.
PNB's announcement late on Monday also sent other shares of state-run lenders lower, reinforcing concerns about the escalating financial cost of the unauthorized loans steered towards billionaire jewelers Nirav Modi and his uncle Mehul Choksi, owner of Gitanjali Gems Ltd.
The scale of the fraud, the biggest to hit an Indian lender, has stunned the country and put the lack of supervisory oversight by the central bank and auditors under the spotlight.
Local media reported on Tuesday the Central Bureau of Investigation, the country's federal police, had raided the offices of law firm Cyril Amarchand Mangaldas, which had represented Nirav Modi for a while.
Cyril Amarchand did not immediately respond to a request for comment, while a CBI media official was not immediately reachable.
Alpesh Mehta, deputy head of research, Motilal Oswal Securities, said markets were still trying to determine the scale of the financial impact since other lenders had given loans to Modi and Choksi based on letters of undertaking (LOUs), or guarantees, provided by PNB.
"People would have never expected these kind of losses in a state-owned bank and it's difficult to ascertain the amount of damage it has caused," Mehta said referring to PNB.
At least a dozen people - six from the bank and six more from Modi's and Choksi's companies - have been arrested, while investigators have seized a number of properties from the two, including jewelry and luxury vehicles.
Both Modi and Choksi, whose whereabouts are unknown, have said they are innocent.
PNB shares were trading down 8.2 percent at 0500 GMT, after earlier hitting its lowest since June 2016.
A sub-index of state-run lenders, the PSU Bank index , was down 2.5 percent, while top-ranked State Bank of India was down 2 percent. (Reporting by Arnab Paul in BENGALURU and Abhirup Roy in MUMBAI Writing by Rafael Nam Editing by Raju Gopalakrishnan)