U.S. authorities have confirmed that a Chinese state-sponsored hacker breached the U.S. Treasury Department, accessing government employees’ workstations and unclassified documents.

President Joe Biden’s administration confirmed this development on Monday, according to the New York Times.

In a letter from the Treasury Department to lawmakers, the incident was flagged on December 8 by BeyondTrust, a third-party software service provider. The hacker had obtained a security key that enabled remote access to certain treasury workstations and the documents stored on them.

“Based on available indicators, the incident has been attributed to a China state-sponsored Advanced Persistent Threat (APT) actor,” the letter reads. “In accordance with Treasury policy, intrusions attributable to an APT are considered a major cybersecurity incident.”

The U.S. Treasury Department is working with the FBI, intelligence agencies, and other investigators to assess the impact of the breach, while the compromised service has since been deactivated, and there is no evidence suggesting that the Chinese actor still has access to treasury information.

This comes from a major cyberattack following previous revelations of China’s penetration into U.S. telecommunications where it gained access to the phone conversations and text messages of U.S. officials.

Chinese officials have consistently denied any government involvement in hacking and have engaged in talks with the U.S. to address cybersecurity concerns, according to The Times.