South Korean investigators have blamed North Korean hackers for the cyberattack on Seoul's establishments, including its nuclear facility in December last year.
The aim of the cyberattack that emerged from Pyongyang was to cause "social unrest and agitating the people," Seoul said.
According to the Yonhap news agency, an unnamed investigator who is part of the probing team, said: "We've learned that the malicious code used in the e-mail attack in December was similar to what North Korean hackers usually employ. Multiple Internet protocol addresses used for the attacks were found to be based in North Korea."
South Korea had said the country's Korea Hydro and Nuclear Power Company witnessed a high-profile computer-generated attack in December by an unidentified hacker. The perpetrator went on to post key information and blueprints of the nuclear facility immediately after the incident.
A similar strike was reported last week when the attacker published classified documents relating to the advance power reactor online, demanding money in return.
"A total of 94 [pieces of] data were compromised. But they are mostly for training and education and far from critical," said prosecutor Choi Yun-soo.
"We've reached the conclusion that the crime was committed by a group of North Korean hackers seeking to stir up unrest and agitation in our country," the investigating team said.
This is not the first time North Korea has attempted to or succeeded in penetrating South Korean systems in similar hacks.
Pyongyang is believed to run a special online army consisting of thousands of trained expert hackers mounting relentless attacks against its adversaries.