The U.S. officials said there is no evidence the hackers breached or disrupted the core systems controlling operations at the plants, so the public was not at risk. Rather, they said, the hackers broke into systems dealing with business and administrative tasks, such as personnel…

The campaign marks the first time Russian government hackers are known to have wormed their way into the networks of American nuclear power companies, several U.S. and industry officials said. And the penetration could be a sign that Russia is seeking to lay the groundwork for more damaging hacks.

The National Security Agency has detected specific activity by the Russian spy agency, the FSB, targeting the energy firms, according to two officials. The NSA declined to comment. The intrusions have been previously reported but not the attribution to Russia by U.S. officials.