[ubernostrum]

If they ask "do you know any foreigners" and you say no, and then they can prove you know a foreigner, that's it: they've got you guilty of a federal felony (lying to an FBI agent about materially relevant matters in the investigation), and since there were two FBI agents one will act as the witness for the other. Then they don't need to investigate anything else; your career is over and you're going to prison.

[smcl]

That's weird, it sounds like this technique is designed to just find someone guilty of something - not the perpetrator of a specific crime, just sweep up a guilty person. Like is the goal of an FBI agent or Policeman simply to be tricksy enough to make someone slip up, so they can sling them in jail?

[Swizec]

But why would a diplomat whose job it is to talk to foreigners say they don't know any?

[TheSpiceIsLife]

Who doesn't know any foreigners?

[CmdrSprinkles]

As mentioned in the other path, it is partly about tripping people up.

The diplomat already provided a list of all known foreign nationals that is supposed to be kept somewhat up to date. If foreign nationals are known that aren't on that list, it raises a red flag.

Why didn't you mention you banged this person? Why are you doing business with that person? Were the questions that this third person asked pressing enough that they should have been added to the list?

It is one of those things that makes perfect sense if you have even a basic understanding of these kinds of processes but that makes for a good narrative for the masses who don't.