Hackers linked to Beijing breached the emails of government officials and stole 60,000 emails from the U.S. State Department, a Senate staffer told Reuters Wednesday.

The emails were stolen from 10 State Department email accounts, according to the source, who works for Senator Eric Schmitt and was present during a briefing by State Department IT officials. Nine of those accounts belonged to people working on East Asian and Pacific affairs and one on Europe, per an email containing briefing details from the staffer.

The hack was first reported in July, and compromised a Microsoft engineer’s device, per Reuters. The breach appeared to hone in on a select number of senior officials managing the U.S.-China relationship. According to the outlet, U.S. officials and Microsoft said that since May, Chinese state-linked hackers had accessed the email accounts at around 25 organizations, including the U.S. Commerce and State Departments. It is not clear what the content of the emails were.

The email accounts of U.S. Ambassador to China Nicholas Burns, Assistant Secretary of State for East Asia Daniel Kritenbrink, and Secretary of Commerce Gina Raimondo were among those breached.

Security experts and former intelligence officials previously told the Wall Street Journal that the timing of the cyber attack occurred during tense relations between the U.S. and China over the war in Ukraine, reports of China and Cuba negotiations to build a joint military training facility, and the shooting down of a Chinese spy balloon that was found hovering above North America earlier this year.