It's not a crime. You just have to declare it.
Of course, this information is going to be interesting to the police. So if they are the proceeds of crime, it will probably help get you caught.
It's not really any different from banks reporting every deposit of $10K or over. The government wants to track all large transfers of money both in bank accounts and across borders, so that it can identify money flows that are not being reported to the IRS and/or are the proceeds of criminal activity.