It was carried out by hackers who worked for a Russian state-sponsored group previously known as Dragonfly or Energetic Bear, the Journal reported. DHS officials said the hacking campaign is likely to continue.

DHS did not immediately respond to CNBC’s request for comment.

Some companies that were compromised may not yet know they have become a victim in a Russian attack, according to the report. That’s because the hackers used the identities of actual employees to enter the utility networks — complicating efforts to detect the intrusions.