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Three activist groups flew a blimp emblazoned with the words "Illegal Spying Below" over the National Security Agency's data center in Utah in protest against the federal government's mass surveillance programmes.
The one-hour flight over the high-tech facility was launched by the environmental group Greenpeace, digital rights activists the Electronic Frontier Foundation, and a conservative political organization, the Tenth Amendment Centre.
The 41m long blimp, which is owned by Greenpeace, was adorned with a sign that read, "NSA Illegal Spying Below."
In an email to Reuters, the agency declined to comment.
But a spokesman did note there is no restricted airspace over the data centre, housed on the grounds of the Utah National Guard's Camp Williams, in Bluffdale, 37km south of Salt Lake City.
The facility, which the NSA says provides the government with intelligence and warnings about cyber security threats, is thought to be the agency's largest data storage centre.
The blimp protest coincided with the launch of an online campaign that rates members of Congress on actions the activists say either further or stop data collection efforts by the NSA.
Greenpeace said the report cards on the site standagainstspying.org were created by analysing NSA reform bills in Congress and weighting proposals on the degree to which they would end mass data collection.
"Our right to privacy is not a partisan issue. It's a human rights issue," said Michael Boldin, founder of Tenth Amendment Centre, which advocates for decentralized government.
"This coalition gives great hope for the future because it shows that people across the political spectrum can set aside differences to work together."