The United States' National Security Agency (NSA) illegally collected as many as 56,000 emails of US citizens annually between 2008 and 2011, according to declassified documents released by authorities.
The US intelligence agency says that the collections were unintentional and due to technical issues, and that the emails were subsequently destroyed.
The once-classified documents were released by US intelligence agencies on Wednesday as part of an unprecedented White House effort to smooth the uproar following revelations by former contractor Edward Snowden about the extent of secret government surveillance programmes.
US officials say the documents show that intelligence collection programmes that inadvertently intrude on Americans' privacy are found and fixed.
The revelations also raise new questions, however, about operations by the NSA and the oversight of those operations by courts operating under the US Foreign Intelligence Surveillance Act (FISA).
"The court is troubled that the government's revelations regarding the NSA's acquisition of Internet transactions mark the third instance in less than three years in which the government has disclosed a substantial misrepresentation regarding the scope of a major collection program," Judge John Bates of the surveillance court wrote in one of the declassified documents.
More specifically, Bates said in an October 2011 ruling that the court had concluded that the process that resulted in improper collections of the tens of thousands of emails was "in some respects, deficient on statutory and constitutional grounds".
The newly declassified documents can be found at on the website of the Director of US National Intelligence, James Clapper. Clapper authorised the declassification, partly in response to a lawsuit filed by the Electronic Frontiers Foundation, an internet civil liberties and rights group.
The emails represent a small slice of the approximately 250 million email communications targeted for collection by the NSA every year.
Under a separate programme, the NSA also keeps records on millions of phone calls made by US citizens and residents.
According to the documents, nine per cent of the emails - around 22.5 million - are collected from "upstream" sources, which officials familiar with intelligence operations said are cable links belonging to telecommunications companies.
The rest are acquired by the NSA from Internet service providers at the point where they are sent or received.
The roughly 56,000 annual emails in question were from "upstream" sources, the use of which the court deemed to be unconstitutional.
Intelligence officials, speaking on condition of anonymity, defended their practices.
"This is not an egregious overreaching by a greedy agency seeking to spy on Americans. This is a technological problem that resulted in an inadvertent collection of a relatively small number of US person communications," a senior intelligence official told reporters.
US intelligence officials told reporters that the domestic emails were collected under a programme designed to target the emails of foreign terrorism suspects.
The programme does not collect emails because of flagged words such as "bomb". Instead it takes in those mentioning specific addresses, or going to or from particular addresses, one official said.
One way that emails of American citizens can get caught in the net is because the programme captures the screenshot of the person's webmail account that shows a page of emails received or sent, rather than just the one targeted email, he said.
Moreover, when collecting from "upstream" sources, data of a targeted person may be bundled by the internet service provider with other users' data.
"For technological reasons NSA was not capable ... and still is not capable of breaking those down into their individual components," the official said.
Officials said that the NSA had decided to "purge" the material after finding that it had been inadvertently collected.
"When you look at these documents taken as a whole, you'll get a sense for the really effective self-policing that goes on at NSA," an intelligence official said. "Any time you have a large technologically complex operation that involves thousands of people, there be will mistakes, there will be errors."