The NSA has broken privacy laws and illegally spied on US citizens thousands of times, an internal audit and other top secret documents obtained by The Washington Post have revealed.
The documents were handed to The Post by whistleblower Edward Snowden earlier this summer, but the newspaper decided to not make them public until now.
The internal audit, which covers the year preceding May 2012, counted 2,776 incidents which resulted in the unauthorised collection, storage or access to private communications on US soil.
Following an increase of significant violations in 2009, the number of NSA oversight staff was quadrupled, however the documents show that the rate of transgressions increased in 2011 and early 2012.
Many of the incidents were caused by technological or human error which resulted in the collections.
One of the most serious of this kind was in 2008 when a "large number" of calls made from Washington were intercepted when a programme confused the state code 202 for the international dialling code of Egypt, +22. This was not reported to oversight staff.
In what appears to be the most brazen violation of privacy laws by the NSA, in 2011 the agency launched a programme which collected large volumes of international data, regardless of whether the emails originated in the US.
The agency did not seek authorisation for the collection method from the Foreign Intelligence Surveillance Court, which grants NSA agents permission to collect data, first.
When the secret court learnt of the programme, it was determined to be unconstitutional and the agency was forced to shut it down.
Responding to the article, an anonymous NSA official speaking with White House permissions said: "We're a human-run agency operating in a complex environment with a number of different regulatory regimes, so at times we find ourselves on the wrong side of the line."
Alluding to the sheer scale of the US government's surveillance programme, he added: "You can look at it as a percentage of our total activity that occurs each day. You look at a number in absolute terms that looks big, and when you look at it in relative terms, it looks a little different."