Theodore “Ted” Kaczynski, the Harvard–educated mathematician who retreated to a dingy shack in the Montana wilderness and ran a 17–year bombing campaign that killed three people and injured 23 others, died Saturday. He was 81.

Branded the “Unabomber” by the FBI, Kaczynski died at the federal prison medical center in Butner, North Carolina, Kristie Breshears, a spokesperson for the federal Bureau of Prisons, told The Associated Press. He was found unresponsive in his cell early Saturday morning and was pronounced dead around 8 a.m., she said. A cause of death was not immediately known.

Before his transfer to the prison medical facility, he had been held in the federal Supermax prison in Florence, Colorado, since May 1998, when he was sentenced to four life sentences plus 30 years for a campaign of terror that set universities nationwide on edge. He admitted committing 16 bombings from 1978 and 1995, permanently maiming several of his victims.

Years before the Sept. 11 attacks and the anthrax mailing, the Unabomber’s deadly homemade bombs changed the way Americans mailed packages and boarded airplanes, even virtually shutting down air travel on the West Coast in July 1995.

He forced The Washington Post, in conjunction with The New York Times, to make the agonizing decision in September 1995 to publish his 35,000–word manifesto, “Industrial Society and Its Future,” which claimed modern society and technology was leading to a sense of powerlessness and alienation.

But it led to his undoing. Kaczynski’s brother, David, and David’s wife, Linda Patrik, recognized the treatise’s tone and tipped off the FBI, which had been searching for the Unabomber for years in nation’s longest, costliest manhunt.

Authorities in April 1996 found him in a 10–by–14–foot (3–by–4–meter) plywood and tarpaper cabin outside Lincoln, Montana, that was filled with journals, a coded diary, explosive ingredients and two completed bombs.

As an elusive criminal mastermind, the Unabomber won his share of sympathizers and comparisons to Daniel Boone, Edward Abbey and Henry David Thoreau

But once revealed as a wild–eyed hermit with long hair and beard who weathered Montana winters in a one–room shack, Kaczynski struck many as more of a pathetic loner than romantic anti–hero.

Even in his own journals, Kaczynski came across not as a committed revolutionary but as a vengeful hermit driven by petty grievances.

“I certainly don’t claim to be an altruist or to be acting for the ‘good’ (whatever that is) of the human race,” he wrote on April 6, 1971. “I act merely from a desire for revenge.”