More internal documents belonging to Korea Hydro & Nuclear Power were leaked on the Internet on Tuesday, marking the fifth security breach since the middle of this month.

Even though a full-blown search is underway for the source of the leaks, hackers posted 19 internal KHNP documents into four compressed files on Twitter.

Self-described as "president of anti-nuclear reactor group in Hawaii," the hacker or hackers have posted classified documents and photos about reactors on Twitter and Korean portals, calling on Korean authorities to close down the reactors.

The latest leak contains five blueprints of two reactors in Gori and 10 blueprints of two reactors in Wolseong. Other files contain safety codes for nuclear power plants developed by KHNP but not yet in use.

But a KHNP spokesman said the blueprints that have been revealed so far "cannot be used to cripple a nuclear power plant."

The Ministry of Trade, Industry and Energy on Tuesday raised the cyber threat level by one notch from "attention" to "caution."

A government investigation team has asked the FBI for help in tracking the IP address of the U.S.-based Twitter account, while tracing the messages the hacker posted on Korean portals like Nate and Naver.

The hackers seem to have used several IP addresses in Japan and the U.S. as well as Korea. Investigators are not ruling out North Korean involvement considering the hacker's use of North Korean-style wording in the messages posted online.