You have not indicated what you were arrested for, but presumably, your car would have been impounded on the theory that it was evidence or contained evidence related to the matter that you were arrested for.
The police could be holding the car as evidence. They could also be holding it because they intend to seize it as part of an asset forfeiture proceeding, which typically occurs in drug cases. If you are charged with a federal offense, you have or need to have a defense attorney representing you. Ask that person to contact the police to find out the reason for the hold.
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