“Unabomber” Ted Kaczynski was found dead on Saturday morning in his North Carolina prison cell, the Federal Bureau of Prisons said.

Kaczynski, 81, was a convicted domestic terrorist who was arrested in 1996 after placing or mailing bombs that killed three and injured 23 others. The incidents occurred between 1978 and 1995. One went off during an American Airlines flight.

In 1995, Kaczynski demanded newspapers publish a 35,000-word manifesto he had written. The Washington Post and The New York Times agreed at the recommendation of the U.S. Attorney General and FBI director.

After reading the words, Kaczynski’s brother and sister-in-law reported to law enforcement that he might be the bomber.

Kaczynski was a Harvard-educated mathematician. He was living in a Montana shack when he was captured.

A prison spokesperson said he was found dead in his cell around 8 a.m. Friday morning.

Kaczynski was incarcerated in Butner, North Carolina. He had spent the first 20-plus years of his life-in-prison sentence at a Colorado Supermax facility.