'Unabomber' Ted Kaczynski found dead in US prison cell

'Unabomber' Theodore "Ted" Kaczynski - who waged a 17-year bombing campaign before evading capture for two decades - has been found dead in his prison cell.

The 81-year-old was given the name by the FBI after his actions caused the death of three people and injured 23 others.

The Harvard-educated mathematician has been locked up since May 1998, when he was sentenced to four life sentences, plus 30 years for the campaign of terror that set universities nationwide on edge.

He admitted to committing 16 bombings from 1978 and 1995, permanently maiming several of his victims.

Kaczynski died at the federal prison medical centre in Butner, North Carolina, Kristie Breshears, a spokesperson for the federal Bureau of Prisons, told the AP news agency.

He was found unresponsive in his cell early Saturday morning and was pronounced dead around 8am. The cause of death was not immediately known.

Years before the September 11 attacks, the deadly homemade bombs changed the way Americans posted packages and boarded airplanes, even virtually shutting down air travel on the west coast in July 1995.

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Brother's suspicions led to his downfall

In 1995, The Washington Post, in conjunction with The New York Times, published his 35,000-word manifesto, which claimed modern society and technology were leading to a sense of powerlessness and alienation.

But it was this that led to his undoing.

Kaczynski's brother David and David's wife, Linda Patrik, recognised the tone and tipped off the FBI.

The hunt for the Unabomber had been the nation's longest and costliest manhunt.

Authorities in April 1996 found him in a 10ft by 14ft plywood and tarpaper cabin outside Lincoln, Montana, which was filled with journals, a coded diary, explosive ingredients and two completed bombs.

Even in his later years, he remained strongly opposed to technology and was not allowed access to the internet.

Handwritten letters by Kaczynski showed he fell in love with one of his pen pals, Joy Richards, and suggest that they discussed the idea of getting married.

Ms Richards died of cancer in 2006, and some of the most anguished letters describe the pain Kaczynksi felt at her illness, Yahoo reported.