ISOLATED CABIN EXEMPLIFIED A LONELY MAN : DISILLUSIONED WITH LIFE, KACZYNSKI FOUND SOLACE IN MONTANA WILDS.

Byline: Robert D. McFadden The New York Times

It was just a dusty, cobwebbed cabin high in the Rockies, as remote as a cougar's lair. But it suited a man who had always been alone, this genius with gifts for solitude, perseverance, secrecy and meticulousness, for penetrating the mysteries of mathematics and the dangers of technology, but never love, never friendship.

The furnishings were the fragments of his life: the books for companionship and the bunk for the lonely hours, the wood stove where night after night he watched dying embers flicker visions of a wretched humanity, the typewriter where, the authorities say, the justifications for murder had been crafted like numbered theorems.

Theodore John Kaczynski had been a brilliant mathematician at the University of California at Berkeley long ago, when he was only 25. But after teaching two years and publishing papers that dazzled his peers and put him on the tenure track at one of the nation's most prestigious universities, he quit in a tailspin of disillusionment with mathematics - the sole passion of his life, suddenly dead.

Over the years since - nearly half his life - he found a kind of freedom as a backwoods hermit in Montana. The miseries of his boyhood in suburban Chicago, his humiliating undergraduate days at Harvard University, the bitterness of years wasted on graduate studies at the University of Michigan, the images of anti-war turmoil at Ann Arbor and Berkeley, had all dimmed with the passing seasons as the face grew old and the beard gray, as obstinate lines formed at the corners of the silent mouth.

Slowly, too, the instabilities he had taken into the woods - the inability to cope with people, the misreading of intentions, the obsessions and rigidities, the anger lurking behind the calm eyes - deteriorated at last, leaving someone even his family did not recognize on the rare occasions they saw him, or in the hundreds of letters that finally stopped.

``I think that truth from my point of view is that Ted has been a disturbed person for a long time and he's gotten more disturbed,'' David Kaczynski, the only brother of the man arrested last month in the Unabomber investigation, said in a six-hour interview with The New York Times. It was David, the person closest to the suspect, who turned him in.

David recalled the terrible day last October when he first read the Unabomber manifesto, a killer's scholarly vision of humanity enslaved in a nightmare world of technology, and found the echoes of his brother's letters and his essays on science, politics and sociology.

``It was horrible to me that I would be considering my brother to be this person,'' he said. But it was all too possible, he knew, that his brilliant and difficult older sibling, whose increasingly troubled and angry life had hurt and mystified his family for decades, might be the phantom who had already killed three people and maimed 23 others in 18 years.

Throughout an afternoon tinged with regret, David Kaczynski, a gentle 46-year-old social worker and vegetarian, a former teacher and an outdoorsman who once lived for months in a tent, seemed out of place in a Manhattan hotel suite where he spoke.

But he maintained his composure until near the end. Then, as dusk sailed the Hudson and failing sunlight struck the walls with shafts the color of whiskey, his soft voice quavered and tears brimmed in his eyes as he spoke of his family's anguish and of his love for his fallen idol.

``I think I love his purity,'' David said. ``I think he's a person who wanted to love something and unfortunately, again, it gets so complex. He failed to love it in the right way because in some deep way, he felt a lack of love and respect himself.''

Six weeks after Theodore Kaczynski's arrest at the cabin, where, the authorities say, the manifesto's master copy and the typewriter used to create it were found with a mountain of evidence, the suspect has remained as silent in his cell in Helena, Mont., as he had been in his 25-year, self-imposed exile.

But conversations with people who have known him, and the interview with the brother who has been the most intimate observer of a secretive man, have provided a detailed and sweeping portrait of the 54-year-old suspect and his personality, mental problems and tortured relationships.

People who had known Ted as a boy, as a high school and college student, as a professor at Berkeley and as a recluse in Montana, as well as investigators and witnesses in the Unabomber case, have drawn a picture of a man whose life seemed destined to be torn apart - a mathematical genius who rose swiftly to academic heights even as he became an emotional cripple.

It is a funereal portrait of loneliness, obsession and contradictions - a Harvard degree at 20, but no one to call a friend; rising success in one of the nation's top mathematics departments, then total retreat from society; a concern for humanity and nature that led finally, officials say, to a one-man war against technology, and the cold calculation of the death of strangers.

Outside his family, Ted seems never to have had a real friend after boyhood. Lengthy searches in Illinois, Michigan, Massachusetts, Iowa, California, Utah and Montana, where he is known to have lived or visited in the last four decades, uncovered no one after his boyhood who had been anything but a casual acquaintance.

And aside from his mother, who doted on him as a boy, there appears to have been no substantive relationship with a woman in all his life. He dated a girl once or twice after high school graduation, but he ended it with a refusal to brook her Catholic precepts. His last date - with a Chicago-area woman who rebuffed him - took place 18 years ago, just after the first Unabomber explosion.

``What amazes me the most about it is that somehow - if he in fact did kill and maim people - he had put a wall around that part of himself and hide it away or keep it inside,'' David said. ``I think his ability to have a conscience, to have sympathy for people, created for him a people problem he could not solve except by walling those feelings.''

As Ted built his walls of physical and psychological separation, David and others said, he cut himself off from a society in turmoil, from parents who he said cared more about his brain than his happiness, and from a brother who, by marrying, came to represent a kind of betrayer.

And as the years of backwoods solitude passed and his contacts with the world withered, his life came to be expressed in the written words he had always used as weapons - hundreds of letters to his family, to a Mexican acquaintance, and to newspapers and magazines; then rambling tracts on society, and finally, the authorities say, the Unabomber manifesto.

David and his parents had long worried over Ted's anger and wondered at its origins. And David recounted stories - how Ted at 9 months had been hospitalized and denied almost all contact with his parents, and how Ted at 7 years had been left alone to sob in a hospital lobby while his father and grandmother went to the maternity ward where David had been born.

He told how the family had come to believe there was something deeply wrong between Ted and his parents, Theodore R. and Wanda Kaczynski. ``I always had a sense that something was missing,'' he said. ``The bond was never complete there, the way it had been with me.''

As the years passed and the adolescent Ted became a friendless intellectual pariah, there were softball games in which David and boys his age were joined by his brother, who felt comfortable among children five, six and seven years younger.