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The head of the National Security Agency told Congress Wednesday that the intelligence agency launched a test program in 2010 to see if it could track Americans' location en masse, using the signals put out by people's cellphones.

According to NSA director General Keith Alexander, the pilot program, which concluded in 2011, was designed to test whether the captured tracking information could be reconciled with databases of information already gathered by the agency's digital dragnet.

"In 2010 and 2011, NSA received samples in order to test the ability of its systems to handle the data format, but that data was not used for any other purposes and was never available for intelligence analysis purposes," Alexander told the Senate Judiciary Committee Wednesday, during a hearing titled, "Continued Oversight of the Foreign Intelligence Surveillance Act."

But in response to a question from Sen. Ted Cruz (R-Texas) about whether the agency might track Americans' locations as part of future terrorism investigations, Alexander suggested that the agency wouldn't mind revisiting its ability to monitor the location of every cellphone in the United States. "This may be something that may be a future requirement for the country, but it is not right now," he said.

[ Is John McAfee's new Wi-Fi box really NSA-proof? Read John McAfee Wants To Shield You From NSA. ]

But Alexander also noted -- as has been disclosed before -- that the agency does share information on suspects' cellphone numbers with law enforcement agencies. "When we identify a number, we get that to the FBI and they can get probable cause to get location data that they need," Alexander said. "And that's the reason that we stopped [the pilot program] in 2011."

The revelations over the test program triggered related questions from privacy experts. "Who were the guinea pigs for this 'pilot program?' And did they consent to being tracked this way?" asked "Dissent," which is the handle of the privacy advocate and data breach information blogger who maintains privacy site PogoWasRight.com. "If not, where was the legal justification or warrant that permitted this?"

The fact that legislators were learning about the test two years after it happened also lead to questions about whether Congress has adequate oversight of the intelligence agency. "The NSA's attempt to collect this data shows the need for stronger legislative oversight of the agency's activities, but the fact is that federal, state and local law enforcement are already regularly collecting cellphone location information without a warrant," ACLU legislative counsel Christopher Calabrese told The Guardian.

Calabrese also suggested that the revelations should drive Congress to finally make clear what types of privacy rights Americans should expect, especially when it comes to having their location tracked. "Last year a majority of the Supreme Court recognized that location information is sensitive, and we need legislation that respects privacy rights when it comes to Americans' movements," he said.

The revelations over the cellphone tracking pilot program came after a July report revealed that the NSA can track cellphones even when they appear to be switched off. According to information published by The Washington Post, the capability was developed to allow CIA and paramilitary units, as well as clandestine Joint Special Operations Command (JSOC) teams, to use al-Qaeda leaders' cellphones to track them in real time, for the purpose of then killing or capturing them.

Technically speaking, tracking "off" cellphones hinged on the fact that even when apparently deactivated, a phone's baseband processor may remain active, pinging a cell tower every 10 minutes to retrieve SMS messages. As a result, should the NSA or Congress choose to pursue mass cellphone location tracking in the future, nothing short of removing a battery from a phone -- when that's even possible -- would prevent people's cellphones from being tracked.