Theodore “Ted” Kaczynski, the man known as the “Unabomber,” has died in custody at a federal prison medical center in Butner, North Carolina, at age 81.

He was found unresponsive Saturday shortly after midnight. Staff performed life-saving measures, and he was transported to a hospital, the Bureau of Prisons said. Kaczynski was pronounced dead at about 8 a.m., Kristie Breshears, a spokesperson for the Bureau of Prisons confirmed to USA TODAY. A cause of death was not immediately known. 

He was serving a life sentence after sending a series of homemade bombs by mail around the country. He pleaded guilty to setting 16 explosions that killed three people and injured 23 others.

Kaczynski had been transferred from supermax prison in Colorado to a federal prison medical facility in North Carolina in December 2021. Officials didn’t say why he was moved at the time.

USA’s most prolific bomber

The FBI spent nearly two decades trying to hunt down Kaczynski, who became known as America’s most prolific bomber.

His first bomb, which the FBI described as “primitive” went off at a Chicago university in 1978. He went on to send 15 more bombs by mail that mostly targeted scientists. His bombs advanced in their sophistication over the years, authorities said.

Kaczynski killed computer rental store owner Hugh Scrutton, advertising executive Thomas Mosser and timber industry lobbyist Gilbert Murray. California geneticist Charles Epstein and Yale University computer expert David Gelernter were maimed by bombs two days apart in June 1993. A dozen people suffered smoke inhalation from a bomb he placed on an airplane in 1979.