South Korea said Friday it would launch a probe into security systems of major retailer Shinsegae and 24 other companies after private data on some 20 million customers was leaked. The move came a day after police arrested three South Koreans for selling private information, including IDs, passwords and addresses, of more than 20 million compatriots online. The three suspects bought the data from Chinese hackers who are still at large, police said. The Ministry of Public Administration and Security said in a statement that government agencies and police would investigate the 25 companies, to see whether they had protected the private data of customers with security codes. The incident appears to be the country's worst case of personal data leakage. In 2008 private information on some 10 million customers was leaked through hacking attacks on Internet Auction, now eBay's South Korea unit.