An image of the site promoting Def Con 21, a large annual gathering of hackers in Las Vegas. The meeting's leader is asking federal workers to stay away from this year's event. Def Con hide caption

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Def Con

Sean Joyce, right, deputy director of the Federal Bureau of Investigation testifies before the House Select Intelligence Committee on Tuesday. Win McNamee/Getty Images hide caption

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Win McNamee/Getty Images

Deputy Attorney General James Cole

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President Obama glances at Britain's Prime Minister David Cameron (right) during a news conference with European Union officials at the G-8 summit in Enniskillen, Northern Ireland, on Monday. WPA Pool/Getty Images hide caption

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WPA Pool/Getty Images

Edward Snowden, seen during a video interview with The Guardian. Glenn Greenwald/Laura Poitras/EPA/Landov hide caption

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Glenn Greenwald/Laura Poitras/EPA/Landov

FBI Director Robert Mueller is sworn in on on Capitol Hill in Washington, on Thursday. J. Scott Applewhite/AP hide caption

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J. Scott Applewhite/AP

Mueller Responds To Nadler

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U.S. Army Gen. Keith Alexander, commander of the U.S. Cyber Command, director of the National Security Agency (NSA), testifies during a Senate Appropriations Committee hearing on Capitol Hill, on Wednesday. Mark Wilson/Getty Images hide caption

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Mark Wilson/Getty Images

In a 12-minute video on The Guardian's website, Edward Snowden talks about how American surveillance systems work and why he decided to reveal that information to the public. The Guardian hide caption

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The Guardian