Late last year, an unidentified hacker posted blueprints of nuclear power plants and threatened to destroy the facilities while demanding they be shut down.
Announcing the results of their preliminary probe into the case, investigators said the series of incidents "is believed to have been caused by an (unidentified) group of North Korean hackers who "aimed at causing social unrest and agitating the people."
The investigators said the hackers stole the data through hacking e-mails and online communities used by former and current officials of the Korea Hydro and Nuclear Power Co., instead of directly infiltrating the operator's network.
"We've learned that the malicious code used in the e-mail attack in December was similar to what North Korean hackers usually employ," an official of the investigation team said.
"Multiple Internet protocol addresses used for the attacks were found to be based in North Korea," he added.
