A bombshell New York Times report reveals that the Chinese government “systematically dismantled” spying operations by the CIA, killing or imprisoning as many as 20 sources over a two-year period starting in 2010. U.S. officials characterized the effort by Beijing as one of the worst intelligence breaches in decades and the whole thing remains a mystery. Although some are convinced the identity of the sources was revealed due to a mole within the CIA, others say China managed to hack the agency’s system of communication.

With the killings and detentions, China destroyed a network of sources that took years to build. And the CIA has not managed to recover. One of the sources was apparently shot and killed in front of colleagues at a government building in China, a sign of how Beijing wanted to send a message to other potential informants.

U.S. officials first began suspecting something was wrong when the high quality information about the Chinese government started to dry up. The FBI and CIA then set out to analyze what happened as sources continued to vanish. Some immediately suspected a mole within the CIA, and investigators quickly began to focus on a former agency operative who had worked in the China division. But they were never able to gather enough evidence to arrest him. Some continue to resist the mole theory though and say the CIA officers were often sloppy with sources.

Despite the continuing disagreement about what happened, there is little doubt about the effect of the crackdown. “The number of American assets lost in China,” notes the Times, “rivaled those lost in the Soviet Union and Russia during the betrayals of both Aldrich Ames and Robert Hanssen, formerly of the C.I.A. and the F.B.I., who divulged intelligence operations to Moscow for years.”