South Korea blamed North Korea Tuesday for a December cyberattack on an operator of a nuclear power plant, saying that the virus used in the attack was similar to one made by North Koreans.

"The malicious codes used for the nuclear-operator hacking were the same in composition and working methods as the so-called 'kimsuky' that North Korean hackers use," a statement from the Seoul central prosecutors' office said, according to Reuters.

Between Dec. 9 and Dec. 12, hackers sent 5,986 phishing emails containing malicious codes to 3,571 employees of Korea Hydro and Nuclear Power Co. Ltd, prosecutors said.

A Twitter user in December had posted Internet links to Korea Hydro’s internal-data archives and issued demands to prevent further leaks.

No critical data was disclosed and the emails, which also aimed at obtaining remote control access of computers, were largely unsuccessful, investigators said, according to The Wall Street Journal.

The investigators added that they traced the intrusions back to Internet addresses registered by North Korea.  

In December, Pyongyang denied involvement in the cyberattack and called the accusations a ploy to escalate tensions between the North and South.

Last week, a hacker believed to be behind the attacks posted more files on Twitter that may have been taken in the cyberattack, Reuters reports. The Twitter account also demanded money and provided an email address where the user said he could be contacted.