U.S. officials believe Chinese intelligence operatives stole sensitive data—including personal information and U.S. government personnel records—and then used it to expose and surveil undercover CIA agents in Africa and Europe, according to Foreign Policy. The bombshell report published Monday includes interviews with over three dozen current and former U.S. officials, detailing the decade-long spy data war between America and China. According to the report, the CIA first became aware in 2013 that undercover personnel flying into Africa and Europe were being identified by Chinese intelligence. The exposures were believed to be the result of a Chinese intelligence operation that involved combing through and synthesizing information from massive, stolen caches. This “was not random or generic,” one source told FP. “It’s a big-data problem.”

Exposing these CIA operatives, according to the report, was one of several moves by the Chinese government after discovering the CIA had networks in China to gather intelligence. “As they learned these things,” the Chinese realized they “needed to start defending themselves,” a former CIA executive said.