Top U.S. intelligence officials are revealing more about their spying in an effort to defend the National Security Agency from charges that it has invaded the privacy of Americans on a mass scale. Yet the latest disclosure — the NSA tried to track Americans' cellphone locations — has only added to the concerns of lawmakers.

NSA chief Gen. Keith Alexander told Congress on Wednesday that his spy agency ran tests in 2010 and 2011 to see if it was technically possible to gather U.S. cell-site data, which can show where a cellphone user traveled. The information was never used, Alexander said, and the testing was reported to congressional intelligence committees.

Alexander also defended his agency, denying reports that it has mined Americans' social media. He also detailed 12 previously revealed cases of abuse by NSA employees who used the network to spy on a spouse or conduct other unsanctioned missions. He said all the employees, with one exception, were disciplined.

Director of National Intelligence James Clapper joined Alexander in testifying at a Senate Judiciary Committee hearing on proposed reforms to the NSA's surveillance of phone and Internet usage around the world, exposed in June by former NSA systems analyst Edward Snowden.

Congress is considering changes in the Foreign Intelligence Surveillance Act, which some believe allows the NSA too much freedom in gathering U.S. phone and Internet data as part of spying on targets overseas. But neither spy chief spent much time discussing the proposed reforms. Instead, lawmakers questioned them about newly reported abuses.

"We only work within the law," Clapper said. "On occasion, we've made mistakes, some quite significant. ... Whenever we found such mistakes, we've reported, addressed and corrected them."

Alexander described the testing of gathering Americans' cellphone data, but he said the NSA did not use the data collected and does not use that capability now. The agency leaves it to the FBI to build a criminal or foreign intelligence case against a suspect and to track the suspect, he said.

"This may be something that is a future requirement for the country, but it is not right now because when we identify a number, we give it to the FBI," he said. "When they get their probable cause, they can get the locational data."

Alexander said if the NSA thought it needed to track someone that way, it would go back to the Foreign Intelligence Surveillance Court — the secret court that authorizes its spying missions — for approval.

The NSA reported the U.S. cell data tests to the House and Senate intelligence committees, Alexander said, and that the data was never used for intelligence analysis. He did not say how many Americans' cellphones were tracked, however, or why his agency thought it might need that capability even hypothetically.

Only last week, Alexander refused to answer questions from Sen. Ron Wyden, D-Ore., about whether his agency had ever collected or planned to collect such cell-site data, saying it was classified. In his testimony Wednesday, the general said the NSA had released the information in letters to the intelligence committees ahead of the Judiciary Committee meeting.

Wyden was not satisfied with Alexander's answer.

"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.

In his defense of the NSA, Alexander denied a New York Times report Saturday that said the NSA searched social networks of Americans. He acknowledged his agency collects data from social networks and other commercial databases to hunt foreign terror suspects but said it is not using the information to build private files on Americans. He said the operations are only used in pursuing foreign agents and sweeping up information on Americans if they are connected to those suspects by phone calls or other data.

Alexander said that not all social network searches are authorized by the secret FISA court, but he added that the agency's searches are proper and audited internally. The authority flows from an executive order on national security dating to the Reagan administration in 1981, he said, adding: "It allows us to understand what the foreign nexus is."

Alexander called the Times report on the searches "inaccurate and wrong." The Times reported that the NSA was exploiting huge collections of personal data to create sophisticated graphs of some Americans' social connections. The newspaper said the private data included Facebook posts and banking, flight, GPS location and voting records.

Alexander and Clapper also told lawmakers that the government shutdown that began Tuesday is seriously damaging the intelligence community's ability to guard against threats. They said that they're keeping counterterrorism staff at work as well as those providing intelligence to troops in Afghanistan, but that some 70 percent of the civilian workforce has been furloughed.

Clapper said he has tried to keep on enough employees to guard against "imminent threats to life or property" but may have to call more back to work if the shutdown continues.

"The danger here ... will accumulate over time. The damage will be insidious," Clapper said. He raised the specter of treason, saying financial stress could make his intelligence officers vulnerable to being bought off by foreign spies.

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