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The arrest last week of a former CIA officer suspected of spying for China exposed one of the most significant intelligence breaches in American history. But the damage is even worse than first reported, sources familiar with the matter tell NBC News.

A secret FBI–CIA task force investigating the case concluded that the Chinese government penetrated the CIA's method of clandestine communication with its spies, using that knowledge to arrest and execute at least 20 CIA informants, according to multiple current and former government officials.

American officials suspect China then shared that information with Russia, which employed it to expose, arrest and possibly even kill American spies in that country, said the current and former officials, who declined to be named discussing a highly sensitive matter. The possible sharing with Russia has not previously been reported.