A Chinese internet address was the source of a cyber attack on one company hit in a massive network shutdown that affected 32,000 computers at six banks and media companies in South Korea, initial findings indicated yesterday.
It is too early to assign blame - internet addresses can easily be manipulated and the investigation could take weeks - but suspicion for Wednesday's shutdown quickly fell on North Korea, which has threatened Seoul and Washington with attack in recent days because of anger over United Nations sanctions imposed for its February 12 nuclear test.
South Korean regulators said they believed the attacks came from a "single organisation", but they have still not finished investigating what happened at the other companies.
Experts say hackers often attack via computers in other countries to hide their identities.
South Korea has previously accused North Korean hackers of using Chinese addresses to infect its networks.
Seoul believes North Korea runs an internet warfare unit aimed at hacking US and South Korean government and military networks to gather information and disrupt service.
The attack on Wednesday caused computer networks at major banks and top TV broadcasters to crash simultaneously. It paralysed bank machines across the country and raised fears that this heavily internet-dependent society was vulnerable.
Yesterday, only one of the attacked banks, Shinhan, was fully online, officials said.