In the story, the author said that they identified themselves as being with the "federal service." Unless the author forgot to mention it, it would seem as though they never explicitly identified themselves as FBI at any point. In a situation like that, it's almost trivial to influence a subject's perception of what they're dealing with between the badges (whatever they were) and their demeanor. The subject expects you to be FBI agents based on the information available, and thus, you are FBI agents. At least to them.
If that's the case, then they were most likely trying to avoid falling afoul of 18 U.S. CODE ยง 912, but I don't think that'd work out to well for them in any case. In United States v. Lepowitch, the Court held that it only requires "that the defendants have, by artifice and deceit, sought to cause the deceived person to follow some course he would not have pursued but for the deceitful conduct."
If that's the case, then they were most likely trying to avoid falling afoul of 18 U.S. CODE ยง 912, but I don't think that'd work out to well for them in any case. In United States v. Lepowitch, the Court held that it only requires "that the defendants have, by artifice and deceit, sought to cause the deceived person to follow some course he would not have pursued but for the deceitful conduct."