Kookmin Bank, South Korea's largest lender, is under investigation for allegedly falsifying customers' loan agreements, police sources said Sunday.

Thirty people who took out loans with the lender filed complaints with the Financial Supervisory Service and law enforcement agencies after they found signs that bank employees had intentionally tampered with due dates on loans.

Those demanding the probe said the original numbers were scratched out using some kind of sharp object and new dates written into the agreement. This kind of action constitutes forgery of legally binding private papers and is a criminal offense.

Complaints raised include the bank asking people who borrowed money to pay back the principal earlier than what was agreed upon in on the loan contract.

"Those who reported the incidents and bank executives will be brought in for questioning to obtain a clear picture of what happened," a police officer responsible for the case said.

Kookmin Bank, meanwhile, said it confirmed that some workers tampered with loan contracts, but stressed it had nothing to do with the lender itself.

"The actions were carried out by individuals who acted without the bank's knowledge," a spokesperson said. The official said when the bank approved loans or allowed a rollover on repayment dates for existing loans, it mandated set dates that were not followed by the frontline workers handing such transactions.

One banker suspected of carrying out the forgery quit the bank last year, but was reinstated as a contract worker. Others who were responsible for overseeing the loans were reassigned to other posts.

If the allegations are found to be true, Kookmin could face considerable flak, while other banks may come under similar scrutiny. (Yonhap News)