Russia has been accused of hacking encrypted FBI communications in 2010, giving their spies access to the U.S. intelligence services most sensitive communications, in what one official has called a 'stunning' breach.
The move meant Vladimir Putin's operatives in Washington, New York, and San Francisco were able to access both the location of FBI surveillance teams and communications between them, Yahoo News reports.
And it was said to be behind the decision by then President Barack Obama to expel 35 diplomats weeks before Donald Trump took office.
The US government had accused them of 'acting in a manner inconsistent with their diplomatic status' over the hacking of US political groups during the 2016 presidential election.
But new information suggests their actions had been part of the plot which hindered the Bureau from tracking Russian spies in the US and stopped the CIA from contacting their assets overseas.
One former senior official called it a 'stunning' technical breakthrough.
Another said it was a 'lightbulb' moment for the US when the activity was discovered, adding: 'This could be why we haven't seen [certain types of] activity.'
Russia has been accused of hacking encrypted FBI communications in 2010, giving their spies access to the U.S. intelligence services most sensitive communications. Russia's President Vladimir Putin is pictured earlier this month
The Federal Bureau of Investigation (FBI) headquarters in Washington. The hack reportedly meant Vladimir Putin's operatives in Washington, New York, and San Francisco were able to access both the location of FBI surveillance teams and communications between
At the time the Russian government dismissed hacking allegations as absurd but Mr Putin declined to order a tit-for-tat expulsion of American diplomats, claiming he would not 'stoop' to Obama's level.
The Russians are said to have accessed the FBI's encrypted radio systems as well as the backup communications systems.
These breaches were reportedly monitored at the Russian diplomatic compounds on the East Coast seized by the Obama administration in 2016.
It hampered the US' ability to track the movements of Russian spies in America, according to reports.
From secret 'listening posts' it is said the Russians also were able to crack FBI radio communications codes.
The wives of some spies were involved, staffing the annexes.
Compounds in Maryland and New York were allegedly being 'used as signals intelligence facilities' and were closed in 2016.
And they are also said to have used 'mobile listening posts'.
Lightweight but outdated radios left America vulnerable, one official said, adding: 'The amount of security you employ is the inverse of being able to do things with flexibility, agility and at scale.'
One former counterintelligence executive said: 'Eventually they switched to push-to-talk cellphones. The tech guys would get upset by that, because if