South Korea is investigating the possible involvement of North Korea in the recent hacking attack on its nuclear power network, Justice Minister Hwang Kyo Ahn said yesterday during a session of the National Assembly.

His remarks came after investigators said an IP address of a suspected hacker was traced to Shenyang city in China, a known location of North Korean computer experts, according to a report in the Chosun newspaper today.

“We are investigating without ruling out the possibility that North Korea may be behind the attack,” Minister Ahn said.

The leaks of partial blueprints and operating manuals for South Korean reactors began last week on a blog and were later posted to a Twitter account under the profile “president of anti-nuclear reactor group.” The group also demanded Korea Hydro & Nuclear Power Co., the nation’s nuclear plant operator, halt three facilities by today. The latest postings on Twitter were on Dec. 23.

“We’ve been up all night guarding our networks and so far everything seems to be under control,” Kim Tae Seok, a spokesman at Korea Hydro, said by phone today. “The worst scenario will be us manually stopping the reactors but even that won’t be too problematic because we are stable with our power supply.”

After the attack, the government raised its cyber crisis alert level to “Substantial,” the third on a five-step scale, according to the National Cyber Security Center.