They usually arrest people at the airport when they are leaving. It's called "hostage diplomacy"[0]. There is a whole Wikipedia page dedicated to it. [0] https

They usually arrest people at the airport when they are leaving. It's called "hostage diplomacy"[0]. There is a whole Wikipedia page dedicated to it. [0] https://en.wikipedia.org/wiki/Hostage_diplomacy#Iran https://en.wikipedia.org/wiki/Hostage_diplomacy#Iran