A branch of KB Kookmin Bank in Seoul, July 9 / Yonhap

Korea's tax agency has launched a special probe into KB Kookmin Bank, a major lender, financial sources said Thursday.

According to insiders, the National Tax Service (NTS) has sent investigators to the bank's headquarters in western Seoul to secure accounting records needed for a tax audit.

The sources added the investigators are from Investigation Bureau 4 of the NTS' Seoul Regional Tax Office, which is in charge of probing tax evasion by businesses. The bureau is widely referred to as the "Grim Reaper" by the business community.

The probe is expected to run through the end of this year.