The FBI last year used a dossier of allegations of Russian ties to Donald Trump's campaign as part of the justification to win approval to secretly monitor a Trump associate, according to US officials briefed on the investigation.
The dossier has also been cited by FBI Director James Comey in some of his briefings to members of Congress in recent weeks, as one of the sources of information the bureau has used to bolster its investigation, according to US officials briefed on the probe.
This includes approval from the secret court that oversees the Foreign Intelligence Surveillance Act (FISA) to monitor the communications of Carter Page, two of the officials said. Last year, Page was identified by the Trump campaign as an adviser on national security.
Officials familiar with the process say even if the application to monitor Page included information from the dossier, it would only be after the FBI had corroborated the information through its own investigation. The officials would not say what or how much was corroborated.
The dossier first came to light when CNN reported that a summary of it had been presented to President Obama and President-elect Trump back in December by top US Intelligence officials.
Comey's briefings to lawmakers stand in contrast to efforts in recent months by the bureau and US intelligence agencies to try to distance themselves from the dossier.
US law enforcement and intelligence officials have said US investigators did their own work, separate from the dossier, to support their findings that Russia tried to meddle in the 2016 presidential election in favor of Trump.
The dossier alleges that Page met senior Russian officials as an emissary of the Trump campaign, and discussed quid-pro-quo deals relating to sanctions, business opportunities and Russia's interference in the election. Page has denied meeting the officials named in the dossier and says he never cut any political deals with the Kremlin.
During the campaign, he traveled to Russia in July, where he gave a lecture critical of US policy toward Russia. That trip drew the attention of the FBI and raised concerns about Page's contacts with suspected Russian operatives, according to US officials briefed on the matter. Page has said he made the trip independent of the Trump campaign and his speech reflected his own views.
Page has also disputed any wrongdoing and says there was nothing illegal in his interactions with Russian officials. He blames former Obama administration officials for pushing the Russia allegations.
"I look forward to the Privacy Act of 1974 lawsuit that I plan to file in response to the civil rights violations by Obama administration appointees last year," Page said in a statement to CNN. "The discovery process will be of great value to the United States, as our nation hears testimony from them under oath, and we receive disclosure of the documents which show what exactly was done in 2016."
The dossier is a collection of memos compiled by a former British intelligence operative working for political opponents of Trump. The memos purport to describe efforts by Russian intelligence to gather compromising information on Trump. CNN reported earlier this year that both President Obama and then-President Elect Trump were briefed on the dossier's existence in part to ensure that Trump understood what was being circulating among intelligence agencies investigating the dossier, and also to emphasize that Russia sought to compile information damaging to Republicans and Democrats. US investigators say they have corroborated some aspects of the allegations, particularly the conversations between foreign nationals that took place as described in the reports.
Comey hasn't mentioned the dossier in all his briefings to lawmakers, according to people familiar with the briefings. To some of them, he has emphasized that the FBI gathered evidence as part of its investigation to support seeking FISA court approval and to take other steps as part of the probe that began last July, according to the officials briefed of the probe.
The Washington Post was first to report the FISA court warrant targeting Page.
To obtain court permission to target Page, the FBI and Justice Department would have to present probable cause that he was acting as an agent of a foreign power, including possibly engaging in clandestine intelligence gathering for a foreign government. Comey and other top Justice Department officials would have to sign off on the application, which government officials say involves a rigorous review process.
Then-candidate Trump cited Page last year in an interview as one of his advisers on national security matters. He was officially on the campaign foreign policy team from March 2016 to September 2016. Page calls himself a 'junior member of the team" and Trump officials have said he had no influential role with the campaign.
The FBI became interested in Page, the officials say, in part because of concerns about his interactions with suspected Russian intelligence operatives last year. The Justice Department and FBI haven't accused Page of wrongdoing and it's unclear whether any charges could be brought in the investigation.
The FBI already knew about Page because of his role as a witness in the 2013 federal prosecution of an undercover Russian spy. Prosecutors alleged that the spy had tried to cultivate Page as a source. Page denies he knew he was interacting with a Russian spy.
Page was also among those who met with Russia's ambassador, Sergey Kislyak during the Republican National Convention in 2016.