Daniel McGowan was returning to his cell in the Sandstone, Minn., federal prison when he learned he was shipping out. At first he didn’t know where, or why, which is standard procedure for the U.S. Bureau of Prisons.

His destination that day in 2008 was a special unit in the Marion, Ill., federal prison for inmates whose words were considered so dangerous that every phone call, letter and visit required tighter controls than the supervision offered in low-security Sandstone. ...

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