The US 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, which was reviewed by Bloomberg News.

The hacker was able to remotely access certain Treasury workstations and “certain unclassified documents maintained by those users,” the department said in the letter to Senators Sherrod Brown and Tim Scott.

Disclosure of the breach comes as the White House continues to investigate what it says is a vast cyber-espionage campaign against US telecommunications companies by Chinese state-sponsored hackers. On Friday, the White House said nine telecom firms had been impacted by the attacks, which have been attributed to a group Microsoft Corp. nicknamed Salt Typhoon.

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