Serial killer Ted Kaczynski, known as the Unabomber, dead at 81

June 10 (UPI) -- Theodore "Ted" Kaczynski, the notorious serial killer known as the Unabomber, has died, officials said Saturday.

Kaczynski, 81, was found unresponsive in his prison cell around 12:25 a.m. at Federal Medical Center Butner in North Carolina, a federal Bureau of Prisons spokesperson confirmed in a statement to UPI.

Responding staff began life-saving measures and requested emergency medical services, which transferred him to a local hospital where he was pronounced dead.

"The Federal Bureau of Investigation was notified," the BOP spokesperson said. "No staff or other inmates were injured and at no time was the public in danger."

No cause of death has been issued yet for Kaczynski, who had been incarcerated since his arrest in 1996.

He had been in failing health and was transferred in late 2021 to the U.S. Bureau of Prison's FMC Butner.

Before that, Kaczynski had been serving his eight life sentences at the notorious ADX Florence supermax prison in Colorado.

He pleaded guilty in 1998 to sending 16 bombs in the mail that killed three people and injured 23 others between 1978 and 1995.

Law enforcement issued a collective sigh of relief when Kaczynski was arrested on April 3, 1996. FBI agents raided a cabin in Montana where he was holed up.

Before that, the serial bomber demanded that national newspapers publish his hand-written manifesto or he would continue sending bombs through the U.S. Postal Service.

The New York Times and Washington Post both printed his 35,000-word screed in 1995 after the U.S. Attorney General and director of the FBI strongly recommended the two publications do so.

Kaczynski earned a Ph.D. in math from the University of Michigan after attending Harvard at the age of 16.

In addition to the letter bombs, he at times threatened to blow up airliners, according to the FBI.

Current Attorney General Merrick Garland supervised Kaczynski's prosecution in California in 1996.