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 to 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 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 as not a committed revolutionary but 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.”

A psychiatrist who interviewed Kaczynski in prison diagnosed him as a paranoid schizophrenic.

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

Kaczynski hated the idea of being viewed as mentally ill, and when his lawyers attempted to present an insanity defense, he tried to fire them. When that failed, he tried to hang himself with his underwear.

Kaczynski eventually pleaded guilty rather than let his defense team proceed with an insanity defense.

“I’m confident that I’m sane,” Kaczynski told Time magazine in 1999. “I don’t get delusions and so forth.”

He was certainly brilliant.

Kaczynski skipped two grades to attend Harvard at age 16 and had published papers in prestigious mathematics journals. His explosives were carefully tested and came in meticulously handcrafted wooden boxes sanded to remove possible fingerprints. Later bombs bore the signature “FC” for “Freedom Club.”

The FBI called him the Unabomber because his early targets seemed to be universities and airlines. An altitude-triggered bomb he mailed in 1979 went off as planned aboard an American Airlines flight; a dozen people aboard suffered from smoke inhalation.

Kaczynski killed computer rental store owner Hugh Scrutton, advertising executive Thomas Mosser and timber industry lobbyist Gilbert Murray. California geneticist Charles Epstein and Yale University computer expert David Gelernter were maimed by bombs two days apart in June 1993.

Mosser was killed in his North Caldwell, New Jersey, home on Dec. 10, 1994, a day he was supposed to be picking out a Christmas tree with his family. His wife, Susan, found him grievously wounded by a barrage of razor blades, pipes and nails.

“He was moaning very softly,” she said at Kaczynski’s 1998 sentencing. “The fingers on his right hand were dangling. I held his left hand. I told him help was coming. I told him I loved him.”

When Kaczynski stepped up his bombs and letters to newspapers and scientists in 1995, experts speculated the Unabomber was jealous of the attention being paid to Oklahoma City bomber Timothy McVeigh.

A threat to blow up a plane out of Los Angeles before the end of the July Fourth weekend threw air travel and mail delivery into chaos. The Unabomber later claimed it was a “prank.”

The Washington Post printed his manifesto at the urging of federal authorities, after the bomber said he would desist from terrorism if a national publication published his treatise.

Patrik had had a disturbing feeling about her brother-in-law even before seeing the manifesto and eventually persuaded her husband to read a copy at the library. After two months of arguments, they took some of Ted Kaczynski’s letters to Patrik’s childhood friend Susan Swanson, a private investigator in Chicago.

Swanson in turn passed them along to former FBI behavior science expert Clint Van Zandt, whose analysts said whoever wrote them had also probably written the Unabomber’s manifesto.

“It was a nightmare,” David Kaczynski, who as a child had idolized his older brother, said in a 2005 speech at Bennington College. “I was literally thinking, ‘My brother’s a serial killer, the most wanted man in America.’”

Swanson turned to a corporate lawyer friend, Anthony Bisceglie, who contacted the FBI.

David Kaczynski wanted his role kept confidential, but his identity quickly leaked out, and Ted Kaczynski vowed never to forgive his younger sibling. He ignored his letters, turned his back on him at court hearings and described David Kaczynski in a 1999 book draft as a “Judas Iscariot (who) … doesn’t even have enough courage to go hang himself.”

Ted Kaczynski was born May 22, 1942, in Chicago, the son of second-generation Polish Catholics — a sausage-maker and a homemaker. He played the trombone in the school band, collected coins and skipped the sixth and 11th grades.

His high school classmates thought him odd, particularly after he showed a school wrestler how to make a mini-bomb that detonated during chemistry class.

Harvard classmates recalled him as a lonely, thin boy with poor personal hygiene and a room that smelled of spoiled milk, rotting food and foot powder.

After graduate studies at the University of Michigan at Ann Arbor, he got a job teaching math at the University of California at Berkeley but found the work difficult and quit abruptly. In 1971, he bought a 1½-acre parcel about 4 miles outside of Lincoln and built a cabin there without heating, plumbing or electricity.

He learned to garden, hunt, make tools and sew, living on a few hundred dollars a year.

He left his cabin in Montana in the late 1970s to work at a foam rubber products manufacturer outside Chicago with his father and brother. But when a female supervisor dumped him after two dates, he began posting insulting limericks about her and wouldn’t stop.

His brother fired him and Ted Kaczynski soon returned to the wilderness to continue plotting his vengeful killing spree.