Woori Bank headquarters building in Jung-gu, central Seoul. (Woori Financial Group)

South Korean prosecutors raided the headquarters of Woori Bank on Monday as part of an ongoing probe into alleged improper loans linked to family members of former Woori Financial Group Chairman Son Tae-seung.

The Seoul Southern District Prosecutors' Office searched offices at Woori Bank’s central Seoul headquarters, including those of the group chairman and the bank’s CEO, seeking internal documents, payment records and transaction logs tied to the alleged 35 billion won ($25 million) in questionable loans.

The investigation, launched after findings by the Financial Supervisory Service, alleges that Woori Bank extended the loans to businesses affiliated with Son's family members, raising concerns over procedural violations.

Prosecutors have intensified their efforts in recent months. In August, they searched the bank’s loan supervision department and finance centers in Guro-gu and Gangnam-gu, Seoul. A second raid in October targeted Son’s home and the residences and offices of other Woori officials.

The scandal has already led to arrests. In September, Son’s brother-in-law was detained for allegedly forging contracts to secure improper loans. Later, a former high-ranking official at Woori Bank suspected of orchestrating the transactions and a former CEO were also taken into custody.

The case adds to a string of high-profile misconducts at Woori Bank. On Sunday, the bank disclosed a fresh 2.5 billion won financial fraud involving an outsider forging documents during loan processing. Earlier this year, the institution faced scrutiny over a 10 billion won embezzlement by a branch official and another forgery-related fraud amounting to 5.56 billion won.