Four days before a sweeping government surveillance law was set to expire last year, Sen. Dianne Feinstein, chair of the Intelligence Committee, took to the Senate floor. She touted the 2008 law's value by listing some of the terrorist attacks it had helped thwart, including "a plot to bomb a downtown Chicago bar" that fall.

Today, however, the government is refusing to say whether the FISA Amendments Act was used to develop evidence to charge Adel Daoud, a 19-year-old accused of the bomb plot.

Daoud's lawyers said in a motion filed Friday that the reason is simple: The government wants to avoid a constitutional challenge to the law, which governs a National Security Agency surveillance program that has become the focus of national debate over its reach into Americans' private communications.