Russian hackers are reported to have gained access to the control rooms of U.S. electric utilities, potentially causing blackouts.
The Wall Street Journal, citing officials at the Department of Homeland Security, reported that the attacks were first detected in 2016 and continued throughout 2017.
The paper added that campaign was likely ongoing.
The hackers are reported to work for a Russian state-sponsored group previously known as Dragonfly or Energetic Bear.
The DHS is yet to officially comment on the matter and Moscow has denied any involvement in hacking what it called critical U.S. infrastructure.