In a message posted at 3 p.m. Korea Time under Twitter ID @John_kdfifj1029, the anonymous attacker warned Korean citizens to stay away from nuclear reactors, and mocked a cybersecurity drill organized Monday by Korea Hydro and Nuclear Power Co., the power plant operator.
"Dec. 9 will be historically remembered," said the message. That's the date when the power plant operator first detected malicious code in its employees' email.
The message also included a link to zip files purporting to be further internal documents from the power company, uploaded to Dropbox and Pastebin.
Earlier, the self-proclaimed anti-nuclear reactor group representative had leaked internal documents such as floor plans and software program manuals, as well as the personal information of power company employees.
The latest message also threatened that 100,000 more pages of documents would be posted and that a "second round of destruction" would be launched if the Korean government did not close down three nuclear plants by Dec. 25.
Korea Hydro and Nuclear Power Co. confirmed that more of its data had been leaked Tuesday, but declined to comment further. It has not said whether the information was hacked by an outsider or stolen by an insider. The operator, as well as the national police and prosecutors, have started a probe investigating the data breach.
The previously stolen files are non-threatening, non-classified documents, according to the company, but local media criticized the plant operator's efforts to play down the seriousness of the situation, and questioned its security systems.
South Korean police have asked the U.S. Federal Bureau of Investigation to cooperate on the investigation for tracking the origin of the Twitter postings, according to a report by Yonhap News, a local news agency.
The police are also looking into the whether the leak is related to the recent hack of Sony Pictures. The FBI said Friday said North Korea was responsible for that attack. The North Korean government, however, has denied the allegations.
Referring to the hacking at Sony Pictures, South Korean President Park said cyberspace is the new war zone for terrorism and the country needs to be prepared for potential cyberterrorist acts.