Originally Posted by
GUWonder
Isn't the most commonly referred to law that asserts a requirement for US citizens to enter the US on a US passport also the very law that asserts a requirement for US citizens to exit the US on a US passport? Yes.
http://www.law.cornell.edu/uscode/text/8/1185
Technically, 8 USC 1185(b) says "it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he
bears a valid United States passport" (emphasis added).
I don't see a definition of "bears" in that portion of 8 USC, so the common meaning of "carries" should apply; if you have it with you (and, presumably, show it to CBP upon request) when departing the US you would seem to be in compliance whether or not it was the passport you showed the airline at check-in.
DISCLAIMER: I am not a lawyer, and I'm certainly not
your lawyer.