https://www.businessinsider.com/how-china-found-cia-spies-leak-2018-8
In a two-year period starting in 2010, Chinese officials began accurately identifying spies working for the US.
Chinese authorities rounded up the suspects and executed or imprisoned them before their handlers were able to determine what was going on.
“You could tell the Chinese weren’t guessing,” one of the US officials said in the report. “The Ministry of State Security were always pulling in the right people.”
“When things started going bad, they went bad fast.”