You should consult with an immigration attorney near you. Under Section 320 of the Immigration and Nationality Act, 8 U.S.C. Section 1431, it appears that you may well have become a U.S. Citizen already. It is odd that the USCIS official kept your passport. An experienced immigration attorney near you should be able to help you to sort this out. Good luck to you.
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I do not understand why they took your green card and passport. That is very strange. You need to hire a lawyer to contact them on your behalf and find out what's going on. It is highly unlikely you will receive a decision prior to May without a lawyer's assistance. Even with a lawyer's assistance, there is no guarantee you get a decision by May. The most important thing is that you get the right decision, and a lawyer can help you with that.