Hackers working for Russia claimed “hundreds of victims” last year in a giant and long-running campaign that put them inside the control rooms of U.S. electric utilities where they could have caused blackouts, federal officials said. They said the campaign likely is continuing.

The Russian hackers, who worked for a shadowy state-sponsored group previously identified as Dragonfly or Energetic Bear, broke into supposedly secure, “air-gapped” or isolated networks owned by utilities with relative ease by first penetrating the networks of key vendors who had trusted relationships with the power companies, said officials at the Department of Homeland Security.

“They got to the point where they could have thrown switches” and disrupted power flows, said Jonathan Homer, chief of industrial-control-system analysis for DHS.

DHS has been warning utility executives with security clearances about the Russian group’s threat to critical infrastructure since 2014. But the briefing on Monday was the first time that DHS has given out information in an unclassified setting with as much detail. It continues to withhold the names of victims but now says there were hundreds of victims, not a few dozen as had been said previously.