Prosecutors searched the computer center of South Korea's No. 2 lender Kookmin Bank on Tuesday as part of a probe into suspicions surrounding the bank's introduction of a new computer system, prosecution sources said.

During the raid on the computer center in western Seoul, prosecution investigators secured company e-mails exchanged among employees in charge of the mainframe system change, according to the sources.

"We secured e-mails on a limited basis to know what opinions had been exchanged inside the company in the process of changing the bank's main computer system," a prosecution official said, requesting not to be named.

The probe began as the Financial Supervisory Service (FSS) filed a complaint against four key executives of KB Financial Group Inc., the lender's parent, on Monday over the system change.

The four, including the chief of the country's No. 2 banking group by market value, are suspected of underestimating the risk of changing the company's IBM-based computer system to a Unix-based system and scaling down related expenses, according to the financial watchdog.

Prosecutors said they will examine the e-mails to determine if the executives took any bribes in return for helping the new computer system be adopted in KB Financial, the country's fourth-biggest lender by assets, in April.

The search comes as KB Financial Chairman Lim Young-rok is under mounting pressure from the company board to resign over the suspicions. The bank's chief executive officer Lee Kun-ho resigned after receiving a heavy sanction from the financial watchdog.

KB Financial and its units have been plagued by a series of scandals, including massive data leaks and illegal loan expenses. (Yonhap)