30-year-old American Edward Snowden, a contract employee at the National Security Agency, is the whistleblower behind significant revelations that surfaced in June 2013 about the US government's top secret, extensive domestic surveillance programmes. Snowden flew to Hong Kong from Hawaii in May 2013, and supplied confidential US government documents to media outlets including the Guardian.
US defends huge surveillance programmes
'Dozens' of potential terrorist plots shut down by controversial data-gathering programmes
Top US intelligence officials say information gleaned from two controversial data-collection programmes run by the National Security Agency has thwarted potential terrorist plots in the US and more than 20 other countries - and that gathered data is destroyed every five years.
Last year, fewer than 300 phone numbers were checked against the database of millions of US phone records gathered daily by the NSA in one of the programmes, the officials said.
No details of the plots or the countries involved were part of the newly declassified information released to Congress on Saturday and made public by the Senate Intelligence Committee. Officials said they were working to declassify dozens of plots NSA chief General Keith Alexander said were disrupted, to show Americans the value of the programmes, but that they want to ensure they don't inadvertently reveal parts of the US counterterrorism playbook in doing so.
The release of information follows a bruising week for US intelligence officials who testified in Congress, defending programmes unknown to the public - and some lawmakers - until they were revealed by ex-NSA contractor Edward Snowden.
The disclosures have sparked debate and legal action against the Obama administration by privacy activists, who say the data collection goes far beyond what was intended under expanded counterterrorism measures passed by Congress after the terror attacks of September 11, 2001.
Intelligence officials said both NSA programmes were reviewed every 90 days by a secret court authorised by the Foreign Intelligence Surveillance Act. Under the programme, the records - which show information such as the time and length of calls, can be examined only for suspected links to terrorism, they said.
The officials said the phone records programme helped the NSA stop a 2009 al-Qaeda plot to blow up New York City subways.
They say it helped track a co-conspirator of al-Qaeda operative Najibullah Zazi, although it's unclear why the FBI needed the NSA to examine Zazi's phone records, as it would have had the authority to gather those records after naming him as a suspect.