Federal authorities are expressing increased alarm about a long-undetected intrusion into U.S. and other computer systems around the globe that officials suspect was carried out by Russian hackers.

Outgoing Attorney General William Barr said the hack of U.S. government agencies “certainly appears to be the Russians.” In implicating the Russians, Barr was siding with the widely held belief within the U.S. government and the cybersecurity community that Russian hackers were responsible for breaches that have affected multiple government agencies, including the Treasury and Commerce departments. The perpetrators breached computer networks through a popular software product from SolarWinds, a Texas-based company with a customer base in the thousands. Several government agencies, including the Departments of Treasury, State, and Energy, said they were among the victims of the hack, though the full scope of it remains unclear.

SolarWinds has said an "outside nation-state" was behind the breach, and Secretary of State Mike Pompeo told conservative radio host Mark Levin on Friday that "we can say pretty clearly that it was the Russians that engaged in this activity."

But on Saturday, President Trump contradicted his secretary of state, suggesting on Twitter that China could be behind the cyber campaign and accusing the media of blowing the breach out of proportion.

Barr told reporters he agrees with Pompeo's assessment.

"It certainly appears to be the Russians," he said.