“This hack was so big in scope that even our cybersecurity experts don’t have a real sense yet in terms of the breadth of the intrusion itself,” adding that it would take some time to fully vet all the agencies and targets.

The breach appeared to provide President-elect Joe Biden with an immediate headache when he takes office on Jan. 20. His transition team’s executive director Yohannes Abraham told reporters on Friday there would be “substantial costs” and the incoming administration “will reserve the right to respond at a time and in a manner of our choosing, often in close coordination with our allies and partners.”