The United States has an interest in the Federal Reserve Banks as tax-exempt federally created instrumentalities.

http://en.wikipedia.org/wiki/Federal_Reserve_System

Legal status of regional Federal Reserve Banks

The Federal Reserve Banks have an intermediate legal status, with some features of private corporations and some features of public federal agencies. The United States has an interest in the Federal Reserve Banks as tax-exempt federally created instrumentalities whose profits belong to the federal government, but this interest is not proprietary.