South Korea laid blame on North Korea for the latest of a string of hack attacks against nuclear power plants, Reuters reports on Tuesday.
Investigation into the matter has shown that the Internet addresses used in the attack belong to North Korea.
Last week, a hacker believed to be behind the attack on Korea Hydro and Nuclear Power Co Ltd, released more data on Twitter, which was most likely taken during the December attack.
Cyber attacks took place between 9 and 12 December by sending 5,986 phishing emails containing malicious codes to 3,571 employees of the nuclear plant operator.
Korea Hydro and Nuclear Power Co Ltd, which runs South Korea's 23 nuclear reactors, said its computer systems had been hacked but only non-critical data had been stolen.
The hacker threatened on Twitter that he will destroy the reactors.
"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 Seoul central prosecutors' office said.
This is not the first time North Korea has been accused of running nation-state cyber attacks against other countries. The communist state was also accused of orchestrating the attack against Sony Pictures Entertainment.
During that attack, a number of Sony's documents were leaked, including contact information and salaries of many movie stars. A few yet unreleased movies were also leaked, including Fury, starring Brad Pitt.
The hackers, calling themselves Guardians of Peace (#GOP), were enraged by the (yet unreleased) comedy movie in which two journalists are sent to assassinate North Korea's leader, Kim Jong-Un.