The Invention Secrecy Act of 1951 has been used
for more than half a century to restrict disclosure of patent
applications that could be “detrimental to national security.” At the
end of the last fiscal year, no fewer than 5,321 secrecy orders were in effect.
These secrecy orders have been difficult to penetrate and the stories
behind them have usually been left untold. But several inventors whose
work prompted imposition of a secrecy order were interviewed by G.W.
Schulz of the Center for Investigative Reporting.
See his new account in Government secrecy orders on patents keep lid on inventions, April 16, 2013.
Source: http://blogs.fas.org/secrecy/2013/04/invention-cir/