John Walker Lindh, serving a 20-year sentence after fighting for the Taliban in Afghanistan, has been transferred to Supermax.
Prison officials confirmed the transfer from Victorville, California, Thursday.
Prison spokesman Isidro Garcia said Lindh was moved to the facility about 90 miles south of Denver in February for security reasons.
Garcia said he had no other information.
Lindh was captured in November 2001 by American forces sent to topple the Taliban after the September 11th terrorist attacks.
He was charged with conspiring to kill Americans and support terrorists but pleaded guilty to lesser charges, including carrying explosives for the Taliban government.