If you are arriving from Canada with a boarding pass for a connecting flight to a U.S. destination, you must follow the "Connections to U.S." signs.
Please note that if you do not have your boarding pass, you have to get one at the check-in counter for your airline or at the self-service kiosks.
Then you need to go through pre-boarding checkpoint C and then U.S. customs.
Your checked luggage will be automatically sent through the luggage handling system. However, U.S. customs officials have the right to retrieve your luggage for a manual inspection, if necessary.