The Washington Post linked the hack, which occurred over the weekend, to a group working for the Russian foreign intelligence service.

The FBI is currently investigating the group, known among private-sector cybersecurity firms as APT29 or Cozy Bear. The hackers are also believed to have breached the State Department, Joint Chiefs of Staff and the White House networks during the Obama administration.

The latest revelation comes less than a month after President Donald Trump fired Christopher Krebs, the nation’s top cybersecurity official.