You don't mention the origin of your dual citizenship status. Were you a child born abroad of American parent(s) who subsequently came to the U.S. and have you established enough years residency in the U.S. to indicate your intent to maintain your U.S. citizenship as an adult past the age of 18 and have now lived on U.S. soil for a minimum of 5 years? That used to be the rule to meet for certain people having dual citizenship. For anyone in that situation entering the U.S. on their non-U.S. passport, it would be taken as an indication that you considered yourself to belong to the other nationality and U.S. citizenship was automatically lost. I've been told the citizenship laws have been amended regarding that provision that mandated a loss of citizenship in that situation, but I'd really check if I were you with the passport office before you leave the U.S. At the very least, obtain a letter from the medical authorities caring for your father to document that you had to leave the U.S. in an emergency situation at short notice.