Originally Posted by
172pilot
Your dual-citizen daughter MUST use her US passport in the GE machine. The US government doesn't recognize dual citizenship thus your daughter is an American citizen when she crosses the border into the US. The machine will recognize her second passport as it's on file but if she is stopped she will need to produce the US passport.
The CBP explicitly said she was under no obligation to have a US passport, just a valid travel document (which Cdn passport and NEXUS cards are, under various conditions) and proof of eligibility to be in the US (which her birth certificate is). Of course, they would prefer she have a US passport, but they said it was not required, and they deemed her a trusted traveller without hesitation. However, I believe you are right that without the US passport, she can't use the GE machine. (I just wish it was otherwise.)