Prove it!
If it was part of the interview he would have (should have) been asked to provide some to cooperate that statement (a developed source) and that person would have (should have) been contacted for cooperation. Those are called developed sources.
Somebody that contacts the FBI out of the blue is usually investigated because most people don’t go out of their way to report this stuff.
I personally do not trust the FBI to do a fair job, sorry they have lost creditability with me.