WASHINGTON — Russian spies in the U.S. conducted a massive operation to track and collect encrypted FBI radio traffic, but there is no evidence they ever cracked the codes and obtained the contents of the communications, two former senior FBI officials tell NBC News.

Nonetheless, the Russian intelligence success, first reported by Yahoo News, provided Vladimir Putin's government unprecedented insights into the activities of secret FBI surveillance teams tracking Russian operatives in the U.S., the former officials said. The breach occurred sometime around 2010, and was well understood by 2012, the former officials said.

Much of the message traffic the Russians collected was processed in two Russian diplomatic facilities that the Obama administration closed in 2016, citing Russia's interference in the presidential election.

"We knew that they were on to us in terms of radio traffic," one former senior official told NBC News. "They had a huge effort they threw at it. But we never saw content."

Yahoo News cited former officials who said the Russians had access to "likely the actual substance of FBI communications," but the two former officials told NBC News they did not believe that to be true. The two former senior officials said they had seen nothing to suggest Russia successfully decoded encrypted U.S. government communications. Rather, the Russians were able to detect and locate secret FBI radio transmissions, they said.

"What they saw was traffic around certain meetings with people who were talking to them," one former official said.

In some cases, the insights the Russians gleaned from the location and movements of FBI surveillance teams led them to stop meeting with sources in the U.S. the former official said.

The former official added that the FBI and CIA learned of the Russian success through some espionage successes of their own, which he declined to detail.

The Russian operation came at a time when the U.S. was developing its own capability to identify covert Russian communications. From March through May of 2010, FBI agents in New York were able to detect specialized encrypted communications sent from the laptop of a Russian spy, Anna Chapman, to a minivan driven by a Russian government official, according to her indictment.

Chapman was arrested along with nine other Russians, who were accused of acting as a network of sleeper agents sent to live in the U.S. under non-official cover. They were deported to Russia in a spy swap.

It was long known that the Russians were using their diplomatic compounds in Maryland and New York as listening posts, which is why the Obama administration seized them in December 2016, officials said. But the CIA and FBI also learned that wives of Russian diplomats were working in the facilities to process FBI radio traffic, said the former senior official, who had direct knowledge of the matter.

The news of the Russian success comes after revelations that the CIA's method of communicating with its informants had been compromised.

NBC News and other organizations reported in 2018 that a secret FBI–CIA task force investigating the case of an American CIA officer spying for China 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.

Yahoo News then reported in November that Iran also had cracked the CIA's covert communications system, resulting in a cascading crisis that put at risk foreigners around the world who had been recruited by the American spy agency to provide information.

Both of these matters are known to the Congressional oversight committees, officials tell NBC News, but since they are classified, there has been no public accountability.