Butner (North Carolina), June 11 Ted Kaczynski, better known as the Unabomber, was found dead in his prison cell, federal officials confirmed to media.
Kaczynski, 81, killed three people and injured 23 more during a mass mail-bombing spree between 1978 and 1995. He later pleaded guilty to his crimes.
He was sentenced to life without parole in 1996 after evading capture for almost 20 years, the BBC reported.
The Harvard-trained mathematician was eventually caught in a Montana cabin.
He was a man who fascinated America for decades, and he became the focus of numerous TV documentaries.
Kaczynski spent the past three decades held at prisons across the US - most recently at the Federal Medical Centre in Butner, North Carolina.
Prison guards at the facility discovered Kaczynski's body on Saturday morning at around 00:25 local time (04:25 GMT), a spokesperson for the US Bureau of Prisons told the BBC.
His cause of death was not immediately clear.
"Responding staff immediately initiated life-saving measures," the spokesperson said. Kaczynski was then "transported by EMS to a local hospital and subsequently pronounced deceased by hospital personnel".
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