Federal authorities say a hack into U.S. government computer systems poses a grave threat to the country.
Officials believe Russia is behind it.
The Cybersecurity and Infrastructure Security Agency says the hack was hard to find... and will be even harder to undo.
The Department of Energy confirms it was affected.
Several other countries were also victims.
President Trump has not commented on the breach.
President-elect Joe Biden says he'll make dealing with it a quote "top priority."
It appears Microsoft was also affected by the hack.
The company said it found malicious software in its systems.,, which were quickly removed.