The Administrative Office of the U.S. Courts — which manages the federal court filing system — first determined how serious the issue was around July 4, said the first person. But the office, along with the Justice Department and individual district courts around the country, is still trying to determine the full extent of the incident.

It is not immediately clear who is behind the hack, though nation-state-affiliated actors are widely suspected, the people said. Criminal organizations may also have been involved, they added.