Ted Kaczynski, the infamous terrorist known as the Unabomber who left a 17-year trail of death and destruction, died Saturday. He was 81. 

Kaczynski passed away at a federal prison medical center in Butner, North Carolina, a spokesperson for the Federal Bureau of Prisons told The Associated Press. He was pronounced dead around 8 a.m. after being found unresponsive in his cell, the spokesperson said. A cause of death was not revealed. 

Prior to his transfer to North Carolina, Kaczynski had been serving a life sentence at ADX Florence, a federal supermax facility in Colorado considered the most secure prison in the U.S., since 1998. He was transferred out of ADX Florence in 2021 in order to receive medical treatment. 

Born in Chicago, Kaczynski grew up as a child prodigy in mathematics, eventually earning an undergradute degree from Harvard University and a PhD from the University of Michigan. 

In 1978, though, he began mailing a series of bombs as part of a terrorist plot against emerging technologies. Between 1978 and 1995, Kaczynski “mailed or hand-delivered a series of increasingly sophisticated bombs that killed three Americans and injured nearly two dozen more,” according to the FBI’s database page on the Unabomber. His actions “changed the way Americans mailed packages and boarded airplanes, even virtually shutting down air travel on the West Coast in July 1995,” the AP noted. 

The FBI began a massive manhunt for the Unabomber, but was unable to track him down for years. The New York Times and The Washington Post even agreed to publish Kaczynski’s 35,000-page manifesto, “Industrial Society and Its Future,” to try and lure him out of hiding. The FBI finally found Kaczynski hiding in a log cabin in Lincoln, Montana, in 1996. He agreed to plead guilty to murder rather than allow his lawyers to use an insanity plea.