Ted Kaczynski, the man known as the "Unabomber" who killed three people and injured 23 in a 17-year mail bombing spree, has been sent from a maximum-security prison to a centre for inmates with health problems.
Kaczynski has been serving a life sentence without parole at a prison in Florence, Colorado, after his 1998 guilty plea.
But the Bureau of Prisons website lists his location as FMC Butner, a federal medical centre in North Carolina.
A Bureau of Prisons spokeswoman told The Washington Post that Kaczynski was transferred to FMC Butner on December 14. But she declined to provide any details about his condition.
The Colorado prison where Kaczynski has been incarcerated is known as the "Alcatraz of the Rockies".
It is home to other notorious inmates including Mexican drug lord Joaquin "El Chapo" Guzman, the British "shoe bomber" Richard Reid and would-be Al Qaeda suicide pilot Zacarias Moussaoui.
Kaczynski, a Harvard graduate and former mathematics professor, sent 16 mail bombs between 1978 and 1995 in a campaign against modern technology, initially hitting universities and airlines.
The nickname "Unabomber" was derived from the acronym used by the FBI to identify him.
Kaczynski conducted his crimes from an isolated cabin in the woods of Montana where he lived without running water or electricity.
His brother David eventually tipped off investigators to his location when he recognised the writing style used in the Unabomber's anti-technology manifesto published at the FBI's urging in 1995.
In September 1995, in exchange for pledging to stop his bombing campaign, The New York Times and The Washington Post published Kaczynski's lengthy manifesto on the evils of technology.
He was captured in April 1996 and given several life sentences without parole in 1998, in a plea deal that helped him to avoid the death penalty.
Despite a diagnosis of paranoid schizophrenia, Kaczynski was still deemed mentally competent to stand trial.
He tried to fire his lawyers who wanted him to plead not guilty by reason of insanity.
Agence France-Presse contributed reporting
