It is a Federal Crime to use fake or false identification in an attempt to fraudulently gain entry into or from the United States. The seriousness of the charges depends upon the type of identification and how it is used.
Assuming that the identification is a fake Passport, you could face Federal Felony Identity Theft charges with a two year minimum mandatory sentence (possibly ameliorated by safety valve because there are no priors) in addition to time for the Federal obtaining a fraudulent Passport charges. After serving time in Federal Prison you'll be deported unless you can establish that despite the fake identification you are in fact a citizen of the United States.
I assume it is not a passport, likely nothing much, customs may seize it and send you a letter. In a worst case you could be arrested, depends on your luck. I would not overly worry until something happens. Your real problem is that you could get the fake ID and get caught using it.
This is not legal advice but a general comment on society.