re: #295 Nerdy Fish

You do not have to clear US customs to get on a US-bound flight from another country. Customs is done when you are exiting the international terminal; if you are using a US airport as a connecting destination between two international flights, you actually never have to traverse US customs at all. As for letting you board a flight, that is entirely up to the security in the country of origin; US customs checks your passport or visa status on the way in, which is where the inadmissible immigrants would get nabbed.

I don’t believe that is true for Canada. In Canadian airports, US border control maintains a checkpoint that travelers must pass through before boarding.