An important Department of Homeland Security (DHS) statement indicated a foreign power, possibly Russia, was held responsible, the New York Times said.
The DHS report carried the second-highest warning rating, the Times said.
Wolf Creek Nuclear Operating Corp refused to say if the plant was hacked but told there had been “no operational impact” at the plant.
“The reason that is reliable is that the operational network systems are completely separate from the corporate network,” spokeswoman Jenny Hageman told Reuters.
In a joint report with the FBI, a spokesman for the Department of Homeland Security told there was “no indication of a threat to public safety”.
The hackers seemed to be attempting to plan out computer networks for future attacks, according to the DHS report seen by the Times.
They sent highly marked emails to senior engineers at operating firms after the nuclear plants, mimicking job applications but laced with malicious code, the newspaper said.
Officials told the Times that the techniques match those used by Russian specialists linked to earlier attacks on energy facilities.
US investigators involved Russia-based hackers of being behind a 2015 attack that produced blackouts across Ukraine.
Hacking is a tool frequently deployed by nation countries to infiltrate foreign manufacturing networks. The US and Israel reportedly used the now-infamous Stuxnet infection in an attempt to damage an Iranian nuclear facility.
The worm struck the facility’s nuclear centrifuges by overriding the system and developing them to spin much faster than intended, reportedly causing severe physical damage.