Theodore Kaczynski - who came to be known as the "Unabomber" for killing three people and injuring many more in a series of mail bombings over 17 years - has been transferred to a federal prison medical facility known for treating inmates with significant health problems, a Bureau of Prisons spokeswoman said Wednesday.

Kaczynski was transferred on Dec. 14 from the supermax prison in Florence, Colo., where he was serving multiple life-in-prison sentences, to FMC Butner, a federal medical center in North Carolina, said spokeswoman Kristie Breshears.

She declined to provide details of Kaczynski's condition. David Kaczynski, Ted Kaczynski's brother, said he had been told recently by someone who corresponds with his brother that he had been moved to a different facility, but the Bureau of Prisons declined to tell him why that was. David Kaczynski, who tipped the FBI to his brother as a possible suspect, said his brother does not respond to his letters.

Kaczynski, 79, pleaded guilty in 1998 to carrying out a series of mail bombings in a deranged campaign against modern technology.

Living in a cramped, crude cabin in the Montana woods, he eluded investigators for years until his brother recognized some of the themes in a manifesto sent to the New York Times and The Washington Post. The brother then notified authorities that the Unabomber might be Ted Kaczynski.

Among the victims of Kaczynski's 16 bombs were a Yale computer scientist, an astronaut candidate, a Sacramento forestry lobbyist, a public relations executive and the president of United Airlines.

His plea deal allowed Kaczynski to avoid the death penalty, though his sentence did not allow for the possibility of his release. Attorney General Merrick Garland, who worked in the Justice Department in a lower capacity when Kaczynski was arrested and helped supervise the prosecution, has said it was one of the most important cases of his career.