Unibomber Ted Kaczynski, a Harvard-educated mathematician who retired to a run-down cabin in the Montana woods and launched a 17-year bombing campaign that killed three people and injured 23 others, died Saturday. He was 81.

According to Kristie Breshears, a spokesperson for the federal Bureau of Prisons, Kaczynski died at the federal prison medical centre in Butner, North Carolina, after being labelled the “Unabomber” by the FBI. He was discovered unresponsive in his cell early Saturday morning and died at 8 a.m., she added. The reason of death was not immediately determined.

He had been incarcerated in the federal Supermax prison in Florence, Colorado, since May 1998, when he was convicted to four life terms plus 30 years for a terror campaign that had campuses around the country on edge. He admits to carrying out 16 explosions between 1978 and 1995, injuring several of his victims permanently.

Years before the Sept. 11 attacks and the anthrax mailing, the Unabomber’s lethal homemade explosives altered how Americans shipped goods and boarded flights, effectively shutting down air travel on the West Coast in July 1995.

In September 1995, he compelled The Washington Post and The New York Times to make the agonizing decision to publish his 35,000-word manifesto, “Industrial Society and Its Future,” which claimed modern culture and technology were contributing to a sense of impotence and alienation.

But that was his undoing. The tone of the book was recognised by Kaczynski’s brother, David, and David’s wife, Linda Patrik, who alerted the FBI, which had been searching for the Unabomber for years in the nation’s longest and most expensive manhunt.

In April 1996, authorities discovered him in a 10-by-14-foot (3-by-4-meter) plywood and tarpaper shack outside Lincoln, Montana, including journals, a coded diary, explosive materials, and two constructed bombs.

As an enigmatic criminal mastermind, the Unabomber drew admirers and similarities to Daniel Boone, Edward Abbey, and Henry David Thoreau.

But when he was revealed to be a wild-eyed recluse with long hair and a beard who spent Montana winters in a one-room cabin, Kaczynski struck many as a pitiful loner rather than a romantic anti-hero.

Even in his own notebooks, Kaczynski comes across as a bitter hermit driven by petty grievances rather than a devoted revolutionary.

“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 only act out of a desire for vengeance.”

Kaczynski was classified as a paranoid schizophrenic by a psychiatrist who interviewed him in prison.

“Mr. Kaczynski’s delusions are mostly persecutory in nature,” wrote Sally Johnson in a 47-page study. “The central themes involve his belief that he is being maligned and harassed by family members and modern society.”

Kaczynski despised the thought of being labelled as mentally ill, and when his lawyers attempted to use insanity as a defence, he attempted to fire them. When that didn’t work, he attempted to hang himself with his pants.

Rather than allowing his defence team to pursue an insanity defence, Kaczynski eventually pleaded guilty.

“I’m confident that I’m sane,” Kaczynski told Time in 1999. “I don’t have delusions or anything like that.”

He was unquestionably brilliant.

At the age of 16, Kaczynski skipped two grades to attend Harvard and had papers published in major mathematics journals. His explosives were meticulously tested and delivered in handcrafted oak boxes polished to remove any fingerprints. Later bombs had the “FC” abbreviation for “Freedom Club.”

He was dubbed the “Unabomber” by the FBI since his initial targets appeared to be colleges and airlines. An altitude-triggered bomb he delivered in 1979 detonated as planned aboard an American Airlines flight, injuring a dozen passengers.

Hugh Scrutton, the owner of a computer rental company, advertising executive Thomas Mosser, and forestry industry lobbyist Gilbert Murray were all assassinated by Kaczynski. Two days apart in June 1993, bombings injured California scientist Charles Epstein and Yale University computer expert David Gelernter.

Mosser was murdered at his North Caldwell, New Jersey, home on December 10, 1994, the day he was due to go tree-hunting with his family. Susan, his wife, discovered him severely injured by a hail of razor blades, pipes, and nails.

“He was moaning very softly,” she stated during Kaczynski’s sentence in 1998. “His right hand’s fingers were dangling. I took his left hand in mine. I informed him that assistance was on its way. I told him how much I adored him.”

Experts speculated that when Kaczynski increased his bombings and letters to journalists and scientists in 1995, he was resentful of the attention being afforded to Oklahoma City bomber Timothy McVeigh.

A threat to blow up a plane leaving Los Angeles before the end of the Fourth of July weekend disrupted air travel and mail delivery. Later, the Unabomber claimed it was a “prank.”

The Washington Post published the Unabomber’s manifesto at the request of federal authorities, after the bomber stated that if a national publication published his treatise, he would cease terrorism.

Patrik had had an uneasy feeling about her brother-in-law even before reading the manifesto, and she eventually got her husband to borrow a copy from the library. They took some of Ted Kaczynski’s letters to Patrik’s boyhood friend Susan Swanson, a private investigator in Chicago, after two months of arguing.

Swanson forwarded them to Clint Van Zandt, a former FBI behavioural science expert, whose analysis concluded that whoever authored them had also likely penned the Unabomber’s manifesto.

“It was a nightmare,” David Kaczynski, who admired his older brother as a boy, said in a 2005 lecture at Bennington College. “I was literally thinking,’My brother’s a serial killer, the most wanted man in America.'”

Swanson contacted the FBI through a business lawyer buddy, Anthony Bisceglie. During his prior tenure at the Justice Department, now-Attorney General Merrick Garland oversaw the investigation and prosecution.

David Kaczynski wanted his role to be kept private, but his identity was swiftly revealed, and Ted Kaczynski vowed never to forgive his younger brother. He ignored his letters, turned his back on him at court proceedings, and in a 1999 book draught, he called David Kaczynski as a “Judas Iscariot (who)… doesn’t even have enough courage to go hang himself.”

Ted Kaczynski was born in Chicago on May 22, 1942, the son of second-generation Polish Catholics — a sausage manufacturer and a housewife. He was in the school band, collected coins, and skipped sixth and eleventh grades.

His high school classmates thought he was strange, especially after he demonstrated to a school wrestler how to create a mini-bomb that exploded during chemistry class.

His Harvard classmates remembered him as a lonely, scrawny boy with bad personal hygiene and a room that smelled like sour milk, decaying food, and foot powder.

He acquired a position teaching maths at the University of California, Berkeley after finishing his PhD studies at the University of Michigan in Ann Arbour, but he found the work challenging and left abruptly. In 1971, he purchased a 112-acre plot of land approximately 4 miles (6 km) outside of Lincoln and constructed a cabin there with no heating, water, or electricity.

Living on a few hundred dollars a year, he learned to grow, hunt, create tools, and sew.

In the late 1970s, he left his Montana cabin to work with his father and brother at a foam rubber goods business north of Chicago. When a female supervisor abandoned him after two dates, he began writing derogatory limericks about her and wouldn’t stop.

Ted Kaczynski was fired by his brother, and he quickly returned to the wilderness to resume plotting his vindictive murdering rampage.