Security, sabotage, spies at Constitution Center

A Thursday, March 3, 2011 photo shows fragments of the planes that hit the World Trade Center in 2001, displayed at the Spies, Traitors and Saboteurs: Fear and Freedom in America exhibit, during a press preview at the National Constitution Center in Philadelphia. The exhibit is scheduled to run from March 4, to May 30. (AP Photo/Matt Rourke)

March 7, 2011 5:15:59 AM PST
It was one of the worst acts of terrorism in U.S. history, a deadly deed that filled the New York City sky with smoke and debris, thrust the nation into war, prompted restrictive new laws and intensified hostility toward foreigners.