 However, federal sentencing rules forbid pleas to be withdrawn after a sentence has already been executed, and Moussaoui had already waived his rights to appeal.On May 13, 2006, a group of US marshals ordered Moussaoui out of his holding cell in Alexandria, Virginia and flew him, via Conair, from Virginia to Colorado to begin serving his sentence at the supermax United States Penitentiary Administrative Maximum Facility, located in Florence, Colorado.The facility - considered the most secure federal penitentiary—is called the "Alcatraz of the Rockies".