This question already has an answer here:
I am leaving soon to visit my country for emergency purposes and I have sent my US passport application in, but for routine service only and then all of a sudden we decided to go back to our country sooner than expected as soon as we booked the flight, I have called in to expedite it but they have the right to say no, they said. I am also not sure if it will come before my flight. Can I leave with my Canadian passport and let my mother bring my U.S passport to me once it finishes processing and enter with it when I come back to the U.S? and also will the immigration know that my US passport is still processing when I present my Canadian passport when I leave? (that is if I don't receive my passport by the time of my flight) hoping to get answers on this..