The US Treasury Department told lawmakers in a letter back in December that its documents and workstations were accessed by an external party in a security breach. It described the attack as "a major cybersecurity incident" and attributed it to a "China state-sponsored Advanced Persistent Threat actor.

Treasury says hackers accessed “certain unclassified documents” in a “major” breach, but experts believe the attack’s impacts could prove to be more significant as new details emerge.

Presidential campaigns and the US telecommunications network have also been targeted by hackers in recent months.