(Reuters) — A hacker believed to be behind cyber attacks on South Korea's sole nuclear power plant operator released more files on Thursday, but a company official said the data was not believed to have been newly stolen but from previous hacking.

Korea Hydro & Nuclear Power, part of state-run utility Korea Electric Power Corp., said in December its computer systems had been hacked but only noncritical data had been stolen and operations were not at risk.

The hacker had at that time demanded the shutdown of three reactors threatening, in Twitter messages, "destruction" if not.

More files were posted on Twitter on Thursday with a demand for money and an email address where the hacker said he could be contacted, claiming there have been offers from other countries to buy data related to nuclear power plants.

"We don't know how they were leaked, but one thing for sure is that there has been no attack from anti-nuclear groups since December," an official at Korea Hydro and Nuclear Power Co Ltd., told Reuters, referring to the newly posted files.

The company official declined to be identified.

The hacker on Thursday repeated a claim to be head of an anti-nuclear power group.

The operator, which runs the country's 23 nuclear power reactors, said in a statement the files included a blueprint and test data for one of the reactors. All of the documents were unclassified, as was the material leaked in December, it said.

The latest leaked data included material likely collected from external sources and the safety of the company's nuclear operations had not been compromised, the company said.

Reuters has not independently verified the content of the files said to be in a Dropbox account.

Police and prosecution officials investigating the leaks declined to comment.

The nuclear operator said after its systems were hacked in December that security measures against cyber attacks were increased.

Investigators said at the time they did not rule out involvement of North Korea in the cyber attack. The North's state media dismissed that as "fabrication."

The December hacking attacks came after the United States accused North Korea of a serious cyber attack on Sony Pictures Entertainment Inc. and vowed to respond proportionately.