NSA chief Gen. Keith Alexander said the tracking took place in 2010 and 2011 and was authorized under a portion of the Patriot Act and with the knowledge and approval of the secret Foreign Intelligence Surveillance Court.
Those who were tracked were not suspected of any wrongdoing or had any known connection to terrorist elements abroad, The Hill reported Wednesday.
Alexander said the program was halted because the NSA doesn't need to collect the information itself. Instead, Alexander said, the NSA passes phone data to the Federal Bureau of Investigations where agents can determine whether there's probable cause to seek a warrant for cellphone data tracking, including GPS information.
"This may be something that would be a future requirement for the country, but it is not right now because when we identify a number, we can give that to the FBI," Alexander said. "When they get their probable cause, they can get the location data they need."
The pilot plan's disclosure was met with criticism from Sen. Ron Wyden, D-Ore., a persistent NSA critic who charged the agency is violating Americans' privacy. Wyden asked Alexander about the GPS tracking issue during a Senate Intelligence Committee hearing last week and Alexander declined to answer, saying the information was classified.
"After years of stonewalling on whether the government has ever tracked or planned to track the location of law abiding Americans through their cellphones, once again, the intelligence leadership has decided to leave most of the real story secret -- even when the truth would not compromise national security," Wyden said.