Woori Bank, one of the five largest lenders in South Korea, is under investigation over suspicious money transfers totaling 800 billion won ($623 million) as one of the bank employees is standing a separate trial on charges that he stole 61 billion won in the largest bank theft local lenders have seen.

A Woori branch in Seoul had sent the money overseas in the last year in what the bank and authorities suspect to be fraudulent remittances that could involve money laundering, given the transaction volume, which is at least 145 times higher than what a single branch handles annually.

“The bank notified us, and we felt a formal investigation was warranted,” an official at the Financial Supervisory Service said, declining to elaborate on details. The top financial watchdog is still looking into how the 61 million-won theft had gone unnoticed for six years, until the employee turned himself in April.

An official at Woori Bank said the lender had not found any wrongdoing incriminating its employees in its latest fraud scandal.

All the paperwork backing the transactions in question was in order, and internal checks had worked, according to the official, who said the unusually high transaction volume prompted the bank to alert authorities.

The bank had reportedly traced the remittances, all in US dollars, to a cryptocurrency exchange in Korea and found out the transfers had all taken place for import payments. But, officials familiar with handling remittances in a foreign currency said such huge transfers usually involve more than import payments.