Financial regulators Monday launched an inquiry into South Korea's largest banking network after a major technical glitch paralysed its businesses for a week.
Financial Supervisory Service and central bank officials visited the Seoul headquarters of the National Agricultural Cooperative Federation, or Nonghyup, to investigate whether it complied with computer security rules.
The system crash that started on April 12, the cause of which is still unclear, left customers unable to withdraw or transfer money, use credit cards or take out loans.
Records of some of its 5.4 million credit card customers were temporarily deleted, leaving the firm unable to bill customers or settle payments to retailers.
Some services including an advanced cash service were still unavailable Monday. Some 310,000 customers have filed complaints and nearly 1,000 called for compensation.
Nonghyup, which has about 5,000 branches, pledged full compensation for any damages and stressed there was no leak of personal data.
It was the second major glitch at a financial firm this month, after Hyundai Capital said a hacker broke into its computer system and stole customer data.
Hyundai Capital, which has about 1.8 million customers, said it lost data on 420,000 customers such as names, residential registration numbers and mobile phone numbers.
About 13,000 passwords also appeared to have been hacked from customers' loan accounts, said the company which is also under investigation by regulators.
Consumer rights groups said they may file class action suits against the two firms.
"We have already enough people to qualify to file suits, but laws are not favourable to consumers in a case like this," Cho Nam-Hee, chief of the Korea Finance Consumer Federation, told AFP.
Cho said the level of protection that financial firms must by law maintain on its online systems is relatively low, and courts usually impose fairly light punishments.