Theodore “Ted” Kaczynski, known as the “Unabomber,” died today at the age of 81 in federal prison, a spokesman for the US Bureau of Prisons told The Associated Press.

Kaczynski was found dead around 8 a.m. in a federal prison in North Carolina. At the moment the cause of his death is unknown.

He was transferred to the federal prison medical facility in North Carolina after spending two decades in a federal Supermax prison in Colorado for a series of bombing attacks on scientists.

Kaczynski was serving a life sentence without the possibility of bail after his 1996 arrest in a rudimentary cabin where he lived in western Montana. He pleaded guilty to causing 16 explosions that killed three people and injured 23 more in various parts of the country between 1978 and 1995.

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