Woori Financial Group headquarters in Seoul / Yonhap

The prosecution raided the headquarters of Woori Financial Group and its bank subsidiary to search for and seize evidence on Monday in an expanded probe into poorly granted loans of over 35 billion won ($25 million) under the unchallenged reign of the group's former Chairman Son Tae-seung.

This is the latest development in a slew of mismanagement scandals at the troubled financial group, blighted significantly by the overall lack of internal control measures despite continued employee embezzlement and poor oversight of lending protocols.

Further blemishing the group is Sunday's report of 2.5 billion won in poorly granted loans to a customer buying property in March. It raises Woori's irregularity this year to four, among a dozen questionable lax lending practices at the bank over the past few years.

Most at stake is the group's long-awaited acquisition of Tongyang Life and ABL Life, certain to be revisited due to escalating scrutiny from the investigative and financial supervisory authorities.

According to the Seoul Southern District Prosecutors' Office, a team of investigators was sent to the headquarters in central Seoul.

They searched for records of loans, approvals and electronic data at the bank's loan department, offices of the bank CEO Cho Byung-kyu and group Chairman Yim Jong-yong.

It came less than three months after the previous one in August on the bank headquarters and branches, as well as the homes of four figures involved. Included were the homes of Son and a circle of his then-loyal subordinates.

The development followed the conclusion of the Financial Supervisory Service (FSS) that the remaining outstanding balance of troubled loans came to 30.4 billion won, as of July 19. Over 88 percent, or 26.9 billion won, is backed by collateral but is nonperforming. Up to 15.8 billion won will be unrecoverable, according to Woori Bank.

The investigation gained momentum after the prosecution imposed an overseas travel ban on Son, identifying him as a suspect rather than a witness. Three Son loyalists under suspecion of expediting the loans have been indicted on charges of bribery over the past two months. Whether the former chairman will be summoned for questioning before a potential indictment remains to be seen.

The FSS extended the inspection period for Woori by a week, citing the need for tightened monitoring in light of the latest 2.5 billion won loan irregularity.

However, the scope of the scrutiny is highly likely to include the acquisition of two life insurers, focusing on capital adequacy ratios, among other compliance issues.

The bank said the 2.5 billion won loan was granted to a customer who falsified records on the collateral to increase the amount borrowable.

The bank conducted an investigation after it was made aware of the incident. It plans to file a criminal complaint against the customer soon.