Infiltrating the U.S. Treasury and Commerce departments, Russian-linked hackers have been linked to a cyberattack by the FBI and Cybersecurity and Infrastructure Security Agency.

According to reports, the cyberespionage campaign is one of the largest hacks against US government agencies over the last five years. Russia is said to be behind the cyberattacks, according to sources close to the investigation.

Investigators believe that so far, internal email traffic at the departments has been monitored. In connection with the attack on U.S. agencies, there has also been a data breach at the cybersecurity firm FireEye.

FireEye officials released a statement on the data breach, saying they have “identified a global campaign that introduces a compromise into the networks of public and private organizations through the software supply chain.”

Officials said the data breach is linked to SolarWinds server software used by multiple U.S. federal agencies and Fortune 500 companies. SolarWinds has about 300,000 customers around the world.

Multiple organizations have been identified as potentially compromised by FireEye going back to the spring of this year.

“Our analysis indicates that these compromises are not self-propagating; each of the attacks require meticulous planning and manual interaction. Our ongoing investigation uncovered this campaign, and we are sharing this information consistent with our standard practice,” officials said.

The Russian foreign ministry posted a statement to Facebook saying the allegations were “unfounded attempts of the U.S. media to blame Russia for hacker attacks on U.S. governmental bodies.”

The statement went on to state “Russia does not conduct offensive operations in the cyber domain.”

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