THE key source for the anti-Trump "dirty dossier" was the subject of an FBI probe over his suspected links with Kremlin spies, newly-released documents reveal.
Feds opened a counterintelligence investigation into Christopher Steele's secret contact in 2009 and at one stage considered obtaining a warrant to wiretap him, they state.
The news has sparked fears disinformation from Moscow may have been fed to former MI6 spy Steele during his own investigation, say new reports.
Steele was the author of a 35-page dossier - compiled before and after the 2016 election - which contained lurid and unproven allegations about Donald Trump in a Moscow hotel room.
The President has long denied claims his campaign colluded with Moscow during the campaign and a later investigation found there was insufficient evidence of any kind of conspiracy.
News of the historic FBI's investigation was revealed in a letter sent by US Attorney General William Barr to Senate Judiciary Chairman Lindsey Graham.
He was responding to requests from Graham's committee as part of its ongoing investigation into the origins of the controversial Russia investigation.
"You have been asking us to accelerate this process and to provide any additional information relating to the reliability of the work of Christopher Steele and the so-called 'Steele dossier'," Barr wrote.
"A footnote in the Inspector General's report contains information, which up till now has been classified and redacted, bearing on the reliability of the Steele dossier.
"The FBI has declassified the relevant portion of the footnote, number 334, which states that 'the Primary Sub-source was the subject of an FBI counterintelligence investigation from 2009 to 2011 that assessed his or her contacts with suspected Russian intelligence officers'."
Barr also revealed that he has asked federal investigators to prepare a declassified summary of information from that same counterintelligence investigation.
"I have consulted with Mr Durham (US Attorney John Durham) who originally brought this information to my attention in the course of his investigation, and he has informed me that the disclosure of the information will not interfere with his criminal investigation," he added.
“This is the most stunning and damning revelation the committee has uncovered,” Senator Graham later said in a statement after releasing the FBI memo.
Attorney Durham was appointed last year to look into how the FBI and other federal agencies investigated alleged Russian interference and whether the Trump campaign worked with the Kremlin.
Special Counsel Robert Mueller's earlier two-year investigation found insufficient evidence pointing to any kind of conspiracy.
At the time, Trump described the inquiry as an "illegal take down that failed" and said the idea of collusion with Russia was "the most ridiculous thing" he had ever heard.
Steele compiled the unverified dossier on behalf of Fusion GPS - which was hired to conduct opposition research funded by the Democratic National Committee and the Hillary Clinton campaign through law firm Perkins Coie.
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According to the FBI document, Steele's apparent source had contact with suspected Russian intelligence officers in Washington, D.C. in 2005 and 2006, reports the Daily Caller.
The memo says that the intelligence officer invited the man to his office at the Russian embassy where he allegedly told the officer that he was interested in entering the Russian diplomatic corps.
The memo also reveals the FBI spoke to several of the man's associates who said he “persistently” asked them about their knowledge of a particular U.S. military vessel.