Unabomber Ted Kaczynski was found dead in his prison cell, according to the Federal Bureau of Prisons. His cause of death was not immediately known.

Kaczynski, 81, known for orchestrating a series of bombings over a roughly 20-year stretch before being caught, was pronounced dead on Saturday when he was found by employees of the North Carolina prison where he was being held, according to ABC News. 

Three people died over the course of his 16 bombings, and was discovered after a family member recognized his writing as similar to the Unabomber’s manifesto.

According to the FBI, Kaczynski “mailed or hand delivered a series of increasingly sophisticated bombs that killed three Americans and injured nearly two dozen more. Along the way, he sowed fear and panic, even threatening to blow up airliners in flight.”

Kaczynski worked as a mathematics professor in the late 1960s at the University of California, Berkeley. He later resigned and lived in a remote cabin in rural Montana.

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Kaczynski’s first bomb was set off before 1980, but he wasn’t arrested until 1996.

“On April 3, 1996, investigators arrested Kaczynski and combed his cabin. There, they found a wealth of bomb components; 40,000 handwritten journal pages that included bomb-making experiments and descriptions of Unabomber crimes; and one live bomb, ready for mailing,” the FBI explained.

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