The US Foreign Intelligence Surveillance Court (FISC) has deemed the National Security Agency’s email and data programme illegal, according to a declassified 2011 opinion ruling released by the country's government.
The Electronic Frontier Foundation (EFF), a civil liberties watchdog, has been fighting the government in federal court for more than a year to publish the 86-page ruling.
The heavily redacted document criticised the government’s misrepresentation of the scope of the NSA surveillance activities, revealing the NSA had collected thousands of US communications in 2011.
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According to the document, the NSA acquired up to 10,000 "multi-communication transactions” each year that contain at least one wholly domestic communication.
The document also said the NSA had been acquiring more than 250 million internet communications in total each year.
However, a month after the FISC ruled the collection unconstitutional, the NSA adjusted its collection process to filter out wholly US traffic from international traffic, according to US reports.
The EFF said the release of the opinion ruling is just one step in advancing a public debate on the scope and legality of the NSA's domestic surveillance.
“The EFF will keep fighting until the NSA's domestic surveillance program is reined in, federal surveillance laws are amended to prevent these kinds of abuse from happening in the future, and government officials are held accountable for their actions,” the watchdog said.
US President Barack Obama has since promised sweeping reforms designed to limit data collection by the NSA under the Patriot Act.
In a statement following the release of the court opinion ruling, US national intelligence director James Clapper announced he is to set up a group to review the US surveillance capabilities and will issue a report by mid-December.