Washington (CNN) 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.

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