You realise that if the FBI already have the criminal activity documented, then all they are doing in the questioning is giving the person the chance to come clean....
There's also a (very remote) possibility that, during the questioning, the person of interest can present additional facts of which the FBI was unaware, thus changing the criminal activity to non-criminal activity. For a (very wild and completely unbelievable) example, Trump could produce evidence that he'd been working with the CIA on a sting operation against Putin, and all of his suspicious behavior was part of a long con, not against the US, but against Russia. Trump would obviously not have been able to admit to this until the con ended, and his behavior during the long con would be consistent with someone guilty of criminal conspiracy.
I'm throwing this out because an FBI interview is not always a foregone conclusion of guilt; it's often a fact-finding portion of an ongoing investigation. It's a crazily-ridiculous example of how Trump could be dismissed as a person of interest, but it's theoretically possible.
In general, when questioned by the FBI, only four answers to any question are advisable:
Yes (truthfully),
No (truthfully),
I don't recall (truthfully), or,
Plead the 5th (and force the FBI to prove their case)