The FBI knew about the 11 deep-cover Russian spies in the US for nearly a decade before they arrested them last week, according to court documents in the case cited by The Washington Post
The report said that in 2006, investigators searched several of the spies' homes, planted microphones, read their messages, and videotaped exchange meetings.
The spies were allowed to linger for so long to promote valuable gains in counter-intelligence, according to The Washington Post
, whereby the FBI could learn more about Russian espionage activities by studying the spies.
The spies' arrest was only triggered after "something happened that was going to affect them all," a senior law enforcement official on the case was quoted as saying.