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. 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.