Since Trump would have the power to shut down the investigation, the trick was to avoid making him feel threatened by it. Therefore, the strategy was to withhold information that illustrated Trump’s centrality to the investigation, assure him that he was not a suspect, and gently admonish him about the need to respect law enforcement’s independence (on pain of being accused of obstruction).
From the FBI’s perspective, there was no downside to telling Trump he was not a suspect. As Comey acknowledged in his hearing testimony, responding to questions from Senator Angus King (I., Maine), any assurances that Trump was not under investigation were only “as of that moment.” Trump’s status could change instantaneously if the investigation of other people, such as Page, turned up any evidence implicating Trump.
That is why Comey kept deflecting Trump’s entreaties that Comey tell the public Trump was not suspected of wrongdoing. In reality, Comey did suspect the president of wrongdoing and calculated that, at some point, the FBI would find the proof. The director therefore declined to absolve the president publicly, which eventually cost him his job. But Comey only gave the private assurances that the president was not “personally under investigation” in order to discourage Trump from interfering in the probe. The real objective of investigating Page and Manafort was to uncover corrupt ties — if there were any — between Russia and Trump. Thus, Trump was always under investigation even if he was not personally targeted for FISA surveillance.