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Do I have to explain the source of *all* income on my taxes?
This is a case where you sit down with an advisor or two. There are legal, and tax issues.  When you deposit the cash, or buy a car with it, the large cash transaction will trigger a notice to the US Government. So they will eventually find out.  Before you get to that point you need to know what obligations and consequences you will be facing. Because you don't know if it was a gift, or found money, or if the owner will be back looking for you to return it; therefore you need expert advice.