I am flying from San Francisco to Leon Guanajuato via Mexico City, and back by the same route. Do I turn in my FMM form (Immigration form) to the airline agents in Leon or Mexico City? I know upon arrival you clear immigration and customs in the 1st point of entry (Mexico City in my case) but my question is the FMM form.
The airline agents collect the form from all foreign passengers at the gate for the international leg. My experience is that they look at the card when you check in and attach it to your international boarding pass, but they don't actually collect it until you get on the plane to leave Mexico.