Cell towers are constantly tracking the location of mobile phones. And that data, federal courts have ruled, is not constitutionally protected. Steve Greer/iStockphoto.com hide caption

itoggle caption Steve Greer/iStockphoto.com

News of U.S. surveillance in Europe has met with distrust and anger; officials are heading to Washington to discuss matters next week. Here, members of an artists' group paint a mural called "Surveillance of the Fittest" on a wall in Cologne, Germany, on Thursday. Frank Augstein/AP hide caption

itoggle caption Frank Augstein/AP

German Chancellor Angela Merkel (right) talks with Italian Prime Minister Enrico Letta on the second day of an European Council meeting in Brussels on Friday. John Thys/AFP/Getty Images hide caption

itoggle caption John Thys/AFP/Getty Images