(Reuters) -Chinese state-sponsored hackers broke into the U.S. Treasury Department earlier this month and stole documents from its workstations, according to a letter to lawmakers that was provided to Reuters on Monday.

The hackers compromised a third-party cybersecurity service provider and were able to access unclassified documents, the letter said, calling it a "major incident."

According to the letter, hackers "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. With access to the stolen key, the threat actor was able override the service's security, remotely access certain Treasury DO user workstations, and access certain unclassified documents maintained by those users."

After being alerted by cybersecurity provider BeyondTrust, the Treasury Department said it was working with the U.S. Cybersecurity and Infrastructure Security Agency (CISA) and the FBI to assess the hack's impact.

BeyondTrust, the CISA and the FBI did not immediately respond to Reuters requests for comment.