Again for future reference: Fifteen airports outside the U.S. have U.S. customs pre-clearance facilities. Most are in Canada and the Caribbean, allowing flights to small U.S. airports that don't have customs and immigration facilities. Others are at Shannon and Dublin in Ireland and in Abu Dhabi. The list will probably grow in the next couple of years. In those places, you take care of the formalities before take-off and arrive in the U.S. as a domestic passenger.