Ted Kaczynski, known as 'Unabomber,' found dead in federal prison cell at age 81

BUTNER, NC — Theodore “Ted” Kaczynski, the Harvard-trained mathematician who retreated to a shantytown in the Montana wilderness and ran a 17-year bombing campaign that killed three people and injured 23, died Saturday. He is 81 years old.

Branded a “Unabomber” by the FBI, Kaczynski died at the federal prison medical center in Butner, North Carolina, Kristie Breshears, a spokeswoman for the federal Bureau of Prisons, told The Associated Press. He was found unresponsive in his cell on Saturday morning and was pronounced dead at around 8 a.m., he said. Responding staff immediately began rescue operations before requesting EMS transportation to a local hospital, where Kaczynski was pronounced dead by hospital staff.

A source told ABC News that Kaczynski's death is being investigated as a suicide, but there is no official cause of death yet.

Prior to being transferred to a prison medical facility, he had been held at Supermax federal penitentiary in Florence, Colorado, since May 1998, when he was sentenced to four life sentences plus 30 years for a campaign of terror that put universities across the country at risk. He confessed to carrying out 16 bombings from 1978 and 1995, permanently injuring some of his victims.

Years before the September 11 attacks and shipments of anthrax, the Unabomber's deadly improvised bomb changed the way Americans delivered packages and boarded planes, even 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 a 35,000-word manifesto, “Industrial Society and Its Future,” which claimed modern society and technology lead to feelings of powerlessness and alienation.

But that led to his downfall. Kaczynski's brother David and David's wife Linda Patrik recognized the tone of the minutes and alerted the FBI, which has been searching for the Unabomber for years in the country's longest and most expensive manhunt.

Authorities in April 1996 found him in a 10 by 14 foot (3 by 4 meter) plywood and tar paper cabin outside Lincoln, Montana, containing a journal, a coded diary, explosives, and two made-up bombs.

An elusive criminal mastermind, the Unabomber won sympathizers and comparisons with Daniel Boone, Edward Abbey and Henry David Thoreau.

But once exposed as a wild-eyed recluse with long hair and a beard who weathered the Montana winters in a one-room shack, Kaczynski struck many as a pathetic loner rather than a romantic anti-hero.

ABC News contributed to this report.