FBI tracking suspects' cell phones without warrants

"From a Newsweek article reporting that the FBI engages in warrantless surveillance of mobile location data from suspects' cellphones: "[Prosecutors] were using the cell phone as a surreptitious tracking device," said Stephen W. Smith, a federal magistrate in Houston. "And I started asking the U.S. Attorney's Office, 'What is the legal authority for this? What is the legal standard for getting this information?'"