Declassified court documents show a NSA surveillance system illegally gathered up to 56,000 personal emails by Americans with no links to terror suspects annually.
Officials revealed that a judge in the Foreign Intelligence Surveillance Court ruled the programme illegal in 2011.
The US government faces mounting criticism over its surveillance operations after the leaks of whistle-blower Edward Snowden.
The court, whose rulings are normally kept secret, said the NSA may have violated US law for collecting as many as 56,000 emails on an annual basis between 2008 and 2011.
But intelligence officials speaking to reporters anonymously say the scooping of emails was unintentional, blaming it on a technological problem.
The NSA was unable to separate out emails between Americans with no direct connection to terrorism, so the agency was collecting tens of thousands of “wholly domestic communications” every year, the court documents said.
NSA may have violated US law for collecting as many as 56,000 emails on an annual basis between 2008 and 2011
In the ruling, Judge John Bates criticized the NSA over the breach of privacy, marking it as “the third instance in less than three years in which the government has disclosed a substantial misrepresentation regarding the scope of a major collection programme”.
The court found that the data gathering violated the Fourth Amendment of the US Constitution, prohibiting “unreasonable searches and seizures”.
The court’s opinions, which are usually kept secret, were revealed by the government in response to a Freedom of Information request.
Government officials said that the court rulings had been declassified to show that eavesdropping programmes at fault had been found and fixed, highlighting its oversight measures.
The scope of the NSA’s massive surveillance programme, which sweeps up internet traffic and phone records, was exposed in June in leaks to media by former NSA contractor Edward Snowden.
Earlier this month, President Barack Obama promised to be more transparent about US spying programmes, with “appropriate reforms” to guarantee greater oversight.
President Barack Obama has vowed “appropriate reforms” to guarantee greater oversight of controversial US surveillance programmes.
At a White House news conference, Barack Obama proposed “safeguards against abuse”, including amending legislation on the collection of telephone data.
The president also urged allowing a lawyer to challenge decisions by the nation’s secretive surveillance court.
He has been defending the programmes since they were leaked in June.
Barack Obama said on Friday that the US “can and must be more transparent” about its snooping on phone and internet data.
“Given the history of abuse by governments, it’s right to ask questions about surveillance, particularly as technology is reshaping every aspect of our lives,” he told reporters.
“It’s not enough for me as president to have confidence in these programmes,” Barack Obama added.
“The American people need to have confidence as well.”
Barack Obama has vowed appropriate reforms to guarantee greater oversight of controversial US surveillance programmes
Barack Obama unveiled four steps aimed at reassuring Americans about the surveillance:
He said he would work with Congress to reform Section 215 of the Bush-era Patriot Act, which governs the programme that collects telephone records
He directed justice officials to make public the legal rationale for the government’s phone-data collection activities, under Section 215
He proposed allowing a lawyer to check the secretive Foreign Intelligence Surveillance Court, which is accused of essentially rubber-stamping government requests to scour electronic records
He announced the formation of a group of external experts to review all US government intelligence and communications technologies
In response to a question about Edward Snowden, the former National Security Agency contractor who revealed details of the secretive surveillance programmes to media, Barack Obama said: “No, I don’t think Mr. Snowden was a patriot.”
The president went on to criticize Russia, two days after he cancelled a planned summit with President Vladimir Putin next month in Moscow.
Barack Obama said there had been more anti-American rhetoric since Vladimir Putin returned to the Russian presidency, which “played into some of the old stereotypes about the Cold War contest”.
“I’ve encouraged Mr. Putin to think forward as opposed to backwards on those issues, with mixed success,” Barack Obama told reporters, who held the news conference just before going on holiday at Martha’s Vineyard, Massachusetts.
He said that during his photocalls with Vladimir Putin, the Russian leader “has got that kind of slouch, looking like he’s the bored kid in the back of the classroom”. But he said their discussions in private had been constructive.
Barack Obama also said he would not consider it “appropriate” to boycott Russia’s Winter Olympics next year, despite calls by gay rights activists to shun the games because of a recently passed law in that country banning “homosexual propaganda”.
Earlier on Friday, Secretary of State John Kerry and Defense Secretary Chuck Hagel held talks with their Russian counterparts in Washington DC.
John Kerry conceded the US-Russia relationship had been complicated by “the occasional collision” and “challenging moments”.
Russian Foreign Minister Sergei Lavrov also acknowledged problems, but said Moscow preferred to handle their differences like “grown-ups”.
It was revealed that Yahoo’s top lawyers had a courtroom showdown with the National Security Agency after it had demanded information on certain foreign users without a warrant, but the tech giant lost and was forced to hand over the data.
Court documents obtained by the New York Timesshow that Yahoo had initially refused to join the PRISM spying program, insisting that the broad national security requests seeking users’ personal information were unconstitutional.
However, the secret court operating under the Foreign Intelligence Surveillance Act (FISA) sided with the NSA and forced Yahoo’s hand.
Information about Yahoo’s legal battle against the NSA first emerged in a heavily redacted court order, but the name of the company involved had not been released until now.
It was claimed that besides Yahoo, a number of major Silicon Valley companies became part of PRISM, among them Apple, Facebook, Microsoft, Google, YouTube, Skype, AOL and the lesser known Internet company PalTalk, which has hosted a lot of traffic during the Arab Spring and the on-going Syrian civil war.
However, only Facebook and Google have been shown to have worked toward creating “online rooms” in which to share data with the government.
Information about the classified program was leaked last week by NSA systems administrator Edward Snowden, 29, who smuggled the files concerning PRISM on a thumb drive.
Yahoo fought against NSA’s warrantless spying program but lost and was forced by secret court to join PRISM
In 2008, Yahoo argued that NSA’s requests seeking to obtain its users’ private data violated their Fourth Amendment rights against unreasonable searches and seizures, but the judges on the FISA court disagreed with the company’s assessment, calling their concerns “overblown”, the Times reported.
“Notwithstanding the parade of horrible trotted out by the petitioner, it has presented no evidence of any actual harm, any egregious risk of error, or any broad potential for abuse,” the court said, adding that the government’s “efforts to protect national security should not be frustrated by the courts”.
Yahoo’s defeat in court reportedly left other top players in the industry more reluctant to challenge the NSA on surveillance request.
So far, Yahoo, Google and Facebook have all denied their involvement in PRISM.
“Yahoo! has not joined any program in which we volunteer to share user data with the U.S. government,” Yahoo General Counsel Ron Bell wrote in a Tumblr post Saturday.
“We do not voluntarily disclose user information. The only disclosures that occur are in response to specific demands.”
In the wake of the unwarranted spying scandal, a number of top Silicon Valley companies, among them Facebook, Google and Microsoft, have asked the government to overturn a gag order allowing them to publicly disclose national security requests.
Google Inc was the first to go public with its demand for greater transparency, releasing an open letter asking the U.S. Department of Justice for permission to disclose the number and scope of data requests each receives from security agencies, including confidential FISA requests.
Microsoft Corp and Facebook Inc soon followed with similarly worded statements in support of Google.
Last year, the US government issued more than 1,850 FISA requests and 15,000 National Security Letters, which refers to requests filed by the FBI to collect information about Americans.
Between 2008 and 2012, only two of more than 8,500 FISA requests were rejected by the secret court, according to the Electronic Privacy Information Center, a non-profit research group.