The Pueblo Chieftain reports Barry "The Baron" Mills was found dead Sunday, one day after his 70th birthday.
Mills and three other leaders of the Aryan Brotherhood were convicted in 2006 in a federal racketeering case, which included multiple counts of murder, conspiracy and drug trafficking. Mills was serving four life terms at the administrative security U.S. penitentiary in Florence.
Fremont County Coroner Randy Keller says the death does not appear to be suspicious, but an autopsy will be conducted Monday. He says all signs so far indicate a "natural death."
Mills entered the gang at San Quentin in California after being convicted of armed robbery in 1969.