Theodore Kaczynski, who was branded the Unabomber by the Federal Bureau of Investigation, died at 81 on Saturday at his prison cell in United States’ North Carolina, reported the Associated Press.

A former mathematician, Kaczynski was sentenced to four life sentences and 30 years in 1998 after he admitted to committing 16 bombings between 1978 and 1995 that killed three persons and permanently maimed several others. Kaczynski used to build untraceable bombs and deliver them to random targets.

On Saturday, Kaczynski was found unresponsive in his cell and was pronounced dead around 8 am, Kristie Breshears, a spokesperson for the federal Bureau of Prisons, told AP. The cause of death is not clear but some reports speculate that he died by suicide.

In 1979, Federal Bureau of Investigation-led task force, which included the Bureau of Alcohol, Tobacco, Firearms and Explosives and US Postal Inspection Service, was formed to investigate the “UNABOM” case, code-named for the University and Airline Bombing targets involved. This earned him the moniker Unabomber.

Kaczynski had evaded his arrest for nearly two decades but was caught after The Washington Post and The New York Times published his 35,000-word manifesto, “Industrial Society and Its Future,” in September 1995. In this manifesto, Kaczynski had claimed modern society and technology was leading to a sense of powerlessness and alienation.

The FBI had approved the investigating team’s recommendation to publish the essay in hopes that a reader could identify the author. David Kaczynski, Theodore Kaczynski’s brother, recognised the letter’s tone and tipped off the investigating agency. This led to Theodore Kaczynski’s arrest in April 1996.

Kaczynski was found in a plywood and tarpaper cabin outside Lincoln, Montana, that was filled with journals, a coded diary, explosive ingredients and two completed bombs.