The U.S. Treasury Department was hacked by a Chinese state-sponsored actor through a third-party software service provider, according to a letter the agency sent to Congress on Monday.

Treasury described the intrusion as a "major cybersecurity incident,” since it was attributed to a state-sponsored actor, according to the letter.

Treasury was notified on Dec. 8 by a third-party software provider, BeyondTrust, that hackers had gained access "to a key used by the vendor to secure a cloud-based service used to remotely provide technical support for Treasury Departmental Offices (DO) end users,” according to the letter.