South Korean police and government agencies are attempting to determine who is responsible for a malicious act that caused widespread computer outages affecting television channels and banking services.
South Korea's communications commission (KCC) says a distributed denial of services (DDos) attack, a common way to overload computer servers making websites unreachable, was not the reason computers at broadcasters and banks became paralyzed.
The manager of the commission's network information protection team, Lee Seung-won, says there are some initial clues as to what actually happened, based on a quick analysis of data collected from the computer systems at the affected institutions.
Lee says suspected malware was circulated through a software update application known as the patch management system, destroying the primary sector on hard drives containing the code needed for starting the operating system (the master boot record).
At the YTN cable news channel, anchorman Ho Jun-seok told viewers the problem affected the work station from which he was trying to read his scripts.
The newscaster says the computer in front of him, which had been working properly when the newscast started, now is paralyzed.
South Korea's Internet and Security Agency says there is no trace of an attack on the computer systems coming from outside the country.
Workers at broadcasters YTN, KBS and MBC say their computers malfunctioned after midday Wednesday.
South Korea's Financial Services Commission issued a statement saying Shinhan's Internet banking servers went down and that computers at Nonghyup and Jeju banks were hit by a virus that deleted files. The Commission adds that Woori Bank successfully fended off a suspected denial-of-service attack.
There are no reports of South Korean government or military computer networks experiencing any trouble. But South Korea's defense minister raised the alert level for the military's information operations condition after receiving word of the problems affecting civilian networks.
North Korea is blamed for previous cyber attacks on South Korean web sites and computer networks, the largest taking place in 2009 and 2011.
Last week, North Korea blamed the United States and its allies for launching a cyber attack against it. All of the web sites hosted in the country were inaccessible for two days.