Russian hackers were able to penetrate the control rooms of hundreds of U.S. utilities last year as part of a campaign against power company vendors that could be ongoing, The Wall Street Journal reported Monday.

Officials with the Department of Homeland Security (DHS) told the Journal that hackers working for a state-sponsored group known as Dragonfly or Energetic Bear were able to get inside the networks of U.S. utilities to the point that they could have disrupted power service and caused blackouts.

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

The department did not disclose which companies were victimized by the hacks but indicated there were hundreds affected by the breach. Other companies reportedly may still be unaware they were part of the breach because the hackers may have broken in using employee credentials.

The U.S. government had previously accused Russia of staging a multiyear cyberattack campaign against the energy grid and other elements of critical infrastructure in the United States.

It said the effort dated back to at least early 2016 and focused on networks belonging to small commercial facilities with the goal of working up toward larger energy companies.

The newly disclosed details of the Russian campaign comes amid growing concerns about Moscow's efforts to interfere in the 2018 midterm elections.

President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE has endured a week of criticism from Republicans and Democrats after he stood next to Russian President Vladimir Putin in Finland and cast doubt on whether Russia interfered in the 2016 election.

He later walked back the statement and expressed confidence in the intelligence community's conclusion that Russia did interfere, though he added that it could have been others besides Russia that did so.

Director of National Intelligence Dan Coats Daniel (Dan) Ray CoatsFBI chief says Russia is trying to interfere in election to undermine Biden The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Former Intel chief had 'deep suspicions' that Putin 'had something on Trump': book MORE warned shortly before Trump met with Putin that "warning lights are blinking red" to indicate that Russia is preparing to launch another campaign to interfere in U.S. elections.