Originally Posted by
tentseller
CORRECTION, you need to go through US CBP immigration, custom and US TSA security upon international(except Canada) arrival at all US airport even if you are doing an international onward connection.
So does the OP have to, seeing how he's flying in from Canada?
I just realized that I have never made an international-international connection at an U.S. airport so I actually have no idea whether or not this is true.