Ted Kaczynski, the convicted domestic terrorist known as the "Unabomber," was found dead in his prison cell Saturday morning, according to multiple reports. 

Kaczynski was found dead around 8 a.m. at a federal prison medical center in Butner, North Carolina, a Bureau of Prisons spokesperson told the Associated Press. He was found unresponsive in his cell early Saturday morning and was pronounced dead around 8 a.m., the spokesperson said. A cause of death is uncertain.

He had been moved to the federal prison medical facility in North Carolina in December 2021 after spending two decades in a federal Supermax prison in Florence, Colorado, for a series of bombings that targeted scientists.

Kaczynski was serving life without the possibility of parole following his 1996 arrest at the primitive cabin where he was living in western Montana. He pleaded guilty to setting 16 explosions that killed three people and injured 23 others in various parts of the country between 1978 and 1995.

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Theodore 'Ted' Kaczynski on his way to arraignment

Theodore 'Ted' Kaczynski is guided to his arraignment by federal marshals, Helena, Montana, April 4, 1996. He had been arrested in connection with the 'Unabomber' bombings and the deaths those explosions caused.  (Michael Macor/The San Francisco Chronicle via Getty Images)

A Harvard-trained mathematician, Kaczynski sent deadly handmade bombs by mail targeting those he believed were causing the destruction of nature through technological advancement. His campaign of terror — which included the bombing of an American Airlines flight with an altitude-triggered device — changed how Americans sent packages through mail and boarded airplanes.

A 1995 threat to blow up a plane out of Los Angeles before the end of the July 4 weekend threw air travel and mail delivery into chaos. The Unabomber later claimed it was a "prank." The FBI dubbed him the "Unabomber" because his early targets seemed to be universities and airlines.

Over 17 years, Kaczynski led authorities on the longest and costliest manhunt in U.S. history before his eventual capture in 1996.

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Ted Kaczynski

Mugshot of Ted Kaczynski, identidentified as the domestic terrorist known as the Unabomber, April 1996.  ((Photo by Bureau of Prisons/Getty Images))

His motives were described in a 35,000 word manifesto titled, "Industrial Society and Its Future," which he forced the Washington Post to publish in 1995. Dubbed the "Unabomber manifesto" by the FBI, then-Attorney General Janet Reno had pushed for the publication of the screed hoping that a reader would be able to identify its author and alert authorities. 

Indeed, Kaczynski’s brother David and David's wife, Linda Patrik, recognized the writing style and tipped off the FBI. 

On April 3, 1996, Kaczynski was arrested in a primitive cabin 75 miles east of Missoula, Montana. Kaczynski was described as an unkempt loner in the sleepy mountain town who ate rabbits, lived without power and rode his bike to the town’s library. At the time of his arrest, Kaczynski had two completed bombs, explosive ingredients, journals and a coded diary, authorities said.

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Unabomber manifesto

View of the front pages of the New York Times and the Washington Post, both who published a manifesto, by a domestic terrorist known as the Unabomber, September 19, 1995. The entire 35,000 word manifesto was published in response to a demand made three months ago by Unabomber, later revealed to be Ted Kaczynski, who had threatened to kill again if his entire philosophy was not printed.  ( LUKE FRAZZA/AFP via Getty Images)

Kaczynski hated the idea of being viewed as mentally ill and during his trial tried to fire his attorneys when they wanted to mount an insanity defense. He eventually pleaded guilty rather than let his attorneys proceed.

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In his journals released at trial by the government at the request of the victims’ families, Kaczynski described his motive as "simply personal revenge."

Fox News' Stephanie Nolasco and the Associated Press contributed to this report.