They can conduct inspections right at the gate. It’s uncommon, but it happens. Typically when this happens, the cabin crew announces that the CBP is conducting

They can conduct inspections right at the gate. It’s uncommon, but it happens. Typically when this happens, the cabin crew announces that the CBP is conducting inspections outside the gate and asks passengers to get their travel documents ready. Outside the gate CBP officers will instruct passengers to form one or more lines and then inspect everybody’s travel documents. Sometimes they only approach certain people and let most passengers walk by.