In the last episode of , John Levine
said:

Indeed. The itinerary was Europe to Canada to US, and the US had two
intl airports (which I won't reveal for privacy). There was a
"pre-screening" in Toronto, where US ICE was operating in Canada,
where they collected my US landing card and asked the typical
questions, only this time without the check-on luggage in my
possession. They showed a digital photo of my check-on, and asked if
that was my bag. I said "yup", and they sent me on my way.

I thought that since they called it a pre-screening, that I would
still need to do more with customs further along the route, but that
never happened.

Nope, that was the customs inspection. Most US airports have no
immigration or customs facilities, and there is no provision for
customs inspection separate from immigration. As soon as you were
precleared in Toronto, you and your luggage were for all practical
purposes in the United States.

It's also worth noting that you are legally admitted to the US at this
point, which means you cannot be detained without access to a lawyer or
searched without cause on immigration grounds -- This is one of the huge
perks of flying into the US at a pre-clearance location, as it avoids
stepping into the rights-free zone where you have no rights at all.

--
Nothing is fool-proof to a sufficiently talented fool.