Ted Kaczynski, the convicted terrorist known as the “Unabomber”, was found dead in his prison cell early Saturday, according to a Federal Bureau of Prisons spokesperson. He was 81.

Kaczynski was found unresponsive in his cell around 12:30 a.m. ET and transported to a local hospital where he was pronounced dead, said the Federal Bureau of Prisons, according to ABC News.

Kaczynski was previously in a maximum security facility in Colorado but was moved to a federal medical center in Butner, North Carolina, in December 2021 due to poor health.

Kaczynski, who went nearly 20 years without being captured until his arrest in 1996, was considered America's most prolific bomber.

Between 1978 and 1995, he placed or mailed 16 bombs that killed three people and injured two dozen others, according to authorities.

In 1995, before he was identified as the Unabomber, he demanded that newspapers publish a long manuscript he had written, saying the killings would continue otherwise. Both the New York Times and Washington Post published the 35,000-word manifesto later that year at the recommendation of the US Attorney General and the director of the FBI.

Kaczynski's sister-in-law, Linda Patrik, was one of the first to identify Kaczynski as the Unabomber after reading the Unabomber's writing.

Kaczynski went on trial in Sacramento, California, where the key issue was not his guilt but his sanity and whether he would be spared the death penalty. He pleaded guilty to murder in exchange for life in prison without parole in 1998.

The name Unabomber was inspired by the case name UNABOM, which is derived from the UNiversity and Airline BOMbing targets, according to the FBI.