When you think about the PRISM warrantless surveillance program, then the NSA probably pops to mind, but the power to review emails accounts the NSA wanted to collect through PRISM was passed to the FBI in 2008.
The FBI, according to a declassified Justice Department report, reviewed NSA’s PRISM selectors for foreignness – to insure that emails and calls by U.S. citizens are not collected. Any users overseas were presumed to be foreign, not communications of Americans. Former FBI General Counsel Valeria Caproni said there was “no reason to presume that the NSA is not upholding its constitutional duty” regarding Americans, “or otherwise violating the FAA or its own FISA Court-approved targeting procedures.” The heavily redacted 231-page report was obtained by New York Times via a Freedom of Information Act lawsuit.
In October 2009, the FBI requested for “raw 702-acquired data to be dual-routed to the FBI;” that means the agency “started retaining copies of unprocessed communication gathered without a warrant to analyze for its own purposes,” wrote Times’ reporter Charlie Savage. Then “in April 2012, the bureau began nominating new email accounts and phone numbers belonging to foreigners for collection, including through the NSA’s ‘upstream’ system, which collects communication transiting network switches.”