The US National Security Agency (NSA) had improperly collected thousands of e-mails from Americans with no links to terrorism over three years before a special court ruled it unconstitutional.
The release of three secret US court opinions Wednesday came amid escalating public debates over NSA's surveillance programmes, which were first revealed by former defence contractor Edward Snowden in June, Xinhua reported.
Under the programme targeting foreign internet traffic, the NSA collected as many as 56,000 emails and other communications by Americans starting from 2008, but failed to effectively filter out all the communications between them.
The US foreign intelligence surveillance court was notified in 2011 of the agency's improper collection of emails and other online communications.
"For the first time, the government has now advised the court that the volume and nature of the information it has been collecting is fundamentally different from what the court had been led to believe," John D. Bates, a judge on the surveillance court, wrote in his opinion in October 2011.
