David Kaczynski on his agonising decision to inform on his sibling and why, despite their not having spoken for 20 years, he still loves him

Of all the riveting stories that David Kaczynski tells about his brother Ted, the most haunting is the tale of the rabbit in the cage. One summer during their 1950s childhood, spent in a suburb outside Chicago, their father caught a baby rabbit. He put it on display in the backyard in a little cage fashioned out of wood and wire. A crowd of local kids, David among them, gathered round, jostling to get a better view.

Suddenly there was a cry from the back: "Oh, oh, let it go!" The boys turn round to find Ted looking distressed and panicked at the sight of the rabbit visibly trembling in its box. The mood turned instantly from jovial to shame-faced; how funny it had been to be ogling at the tiny animal, and how cruel it seemed now. The father grabbed the box and quickly carried it to a wooded area across the street where he let the little rabbit go, back to the wild.

The story is poignant in part because Kaczynski's brother now lives in a cage built of concrete and reinforced steel within a "supermax" security prison, and in part because of how the story was recalled to David years later. For he had forgotten all about the rabbit for the best part of 40 years, until his mother reminded him of it – on the day he told her that his brother was suspected of being the Unabomber, the "neo-Luddite" murderer who, over 17 years, waged a twisted campaign of mail bombings against targets including American universities and airlines, killing three people and injuring many more. Kaczynski confessed to his mother that he had informed on Ted to the FBI, fearing further atrocities, and that because of his action, his brother could face the death penalty.

Kaczynski had no idea how his mother would react to the news. Would she disown him for betraying his own brother? Would she for ever cast him out of her love? Instead, she took his head in her hands and kissed him. "I know you love Ted, and wouldn't have done that if you hadn't had to," she said. And then she told him the story of the rabbit in the cage. "She told me that story," Kaczynski says, "at the moment Ted was caught, was trapped, knowing perhaps he would end in the fulfilment of his own worst nightmare."

Kaczynski knows a thing or two about the joys and the torture of being a younger brother, and has distilled his experiences of life as the brother of the Unabomber into an essay for a new collection of reflections on brotherly love and rivalry. David calls his chapter Missing Parts – he has had virtually no contact with Ted for the past 20 years and has, in a sense, lost part of himself in the process. By 1996, when Ted was brought into court on 13 counts of bombing and murder, they had already been estranged for seven years. Kaczynski recalls seeing Ted again after all that time: "He walked into the court almost directly towards me, but he never made eye contact. He just turned and sat down with his back to me."

Kaczynski's essay is painful testimony to the ability of brothers to inflict almost unthinkable wounds on each other. Ted cut off all relations with David in 1989; David shopped Ted to the Feds six years later. But it also dwells on the kinder side of brotherhood; on the friendship and loyalty that each bestowed upon the other, and on the love that Kaczynski still reserves for Ted despite his grotesque deeds.

Through the pall of anger and ugliness that descended over his brother, the vicious letters he received from Ted and the rants against technology, Kaczynski still remembers the small acts of kindness and affection that Ted extended to him in their younger years. When David, aged three, couldn't reach the door handle in their home, Ted improvised a new one for him out of an old spool of thread.

Every weekend, the brothers would be driven by their father into the forests outside Chicago, where they would revel in nature (a theme that would build over the years), identifying plants and pitching tents. "Some of the happiest experiences of my life were these with Ted; out of doors, a release from confinement of various kinds," Kaczynski says when we meet in New York. "Growing up, I never doubted my brother's fundamental loyalty and love, or felt the slightest insecurity in his presence."

And yet, from an early age, Kaczynski was aware of something different, something inexplicable and out of place about his big brother. Ted was hyper-smart – everybody knew that. He was a mathematics whizz-kid and destined for Harvard and great things. But he was also a withdrawn, awkward boy who recoiled from social contact.

"When we were young, friends and family would turn up at our house unannounced. My feeling was 'Oh good! Here's Uncle Stanley or our friend Ralph' – but Ted's reaction was the opposite. He saw it as an incursion into his world, and almost in panic would run upstairs to the attic. I remember thinking, why did he have this aversion to people?"

Kaczynski was only seven when he first formulated those doubts into words. "What's wrong with Teddy?" he asked his mother. In reply, she told him that when Ted was just a baby, he had been hospitalised for several days with a rash; the experience of being separated from his parents had, she believed, hurt him deeply with lasting consequences. Then she said something startling to her younger son: "Never abandon Ted, because that's what he fears the most."

And until he faced the awful decision of whether to turn Ted over to the FBI, Kaczynski never did abandon him. In spite of his brother's growing eccentricities, he provided Ted with a social prop. "It almost seemed I was his ticket to having social relationships."

Indeed, it was because of David that Ted ended up in Montana, the rugged north-western state in which he built his now infamous remote wooden cabin. Together they had bought a plot of forest land outside Lincoln, and there Ted constructed what was to become the headquarters of his bombing campaign.

Though David was the socially-adept half of the relationship, he continued to idolise and emulate Ted throughout his youth and well into adulthood. He applied for Harvard, following in Ted's footsteps, but was rejected. Later, he decided to follow Ted's example and go back to the land. When Ted refused to let him build a second cabin on their shared plot in Montana, Kaczynski went instead to a wild part of western Texas where, just like his brother, he lived without running water or electricity for eight years in a cabin he built by hand. They would correspond frequently; two spartan men in their cabin hermitages 1,000 miles apart.

But as time passed, it became clear they were not really communicating, and were, in fact, living in wholly separate wildernesses. Kaczynski's vision of back-to-the-land was a spiritual journey of discovery, towards some inner understanding, whereas Ted's philosophy, his cabinology, was all about getting away from the collective mess of the modern world. There was a despondency, a sorry defeatism in him. "You could call the difference between us one between the left brain and the right brain. Ted was hyper-analytical. It's curious that he rejected technology because his way of thinking was very scientific, very binary."

The moment that crystallised this yawning gulf between them came, paradoxically, at a time when the brothers had never felt more close. It was 1969 and they had spent the whole summer together, travelling huge distances across Canada in search of a plot of land where Ted could begin his anti-civilisation mission. At the end of the trip, as they were driving back to Chicago, they camped overnight in the grasslands of Nebraska. They lay side by side, staring up at the immense night sky stuffed with stars. David felt eager to get home, to familiar things and their mother's home cooking. "I wish we were home," he said.

Ted felt the opposite: "Really? I wish we didn't have to go back," he said.

Later, of course, the distinctions grew stark and ugly. From 1977, Ted began sending his parents angry, blistering letters accusing them of never having loved him. Then, in 1978, Kaczynski ended up sacking his own brother from a factory job in Illinois after Ted began harassing a fellow woman worker, posting crude and offensive limericks about her on the factory wall. The timing was significant – only a few months before, in May 1978, he had posted his first mail bomb to a university professor in Chicago, who was mildly injured in the blast. A year later, he came close to blowing up an American Airlines jet but the bomb failed to detonate.

Over time, Ted's homemade bombs became more sophisticated and powerful, and the first serious injury occurred in 1982 when a university secretary suffered severe burns to her hands. Three people died during the 16-bomb campaign – a computer rental store owner in 1985, an advertising executive in 1994, and (the final target) a timber industry lobbyist in 1995. Another 23 suffered often hideous injuries, the victims having often been selected – by dint of Ted's loathing of technology – from university departments and airlines; hence the moniker Unabomber (University and Airline Bomber).

As the violence escalated, so too did the hostility Ted showed for his family. The final rift came in 1989, when Kaczynski wrote to Ted to tell him he was leaving his cabin retreat in Texas and going to live in New York state with Linda, a childhood friend with whom he had fallen in love. Ted's response was a 20-page letter in which he tore into his brother, accusing him of lacking the integrity to lead a pure life.

"Wow! It was like a metaphorical bomb for me, that he was so hostile," Kaczynski says. "It was at a different level to anything before."

Ted ended the letter by saying that he would have nothing to do with his brother from then on. If there was a family emergency, David was to put a line under the stamp on the envelope, otherwise Ted would just burn the letter unopened. If David abused the privilege of the line under the stamp, by using it for anything other than a genuine emergency, all lines of contact would be terminated for ever.

Kaczynski only once used the line under the stamp, to tell Ted that their father was dying from lung cancer. Ted did reply to that letter. He thanked David for using the line appropriately. He made no mention of their father.

In the end it was Linda, by now Kaczynski's wife, who connected Ted to the Unabomber. She had noticed telling similarities from newspaper accounts. At first Kaczynski had been sceptical, but then in 1995 when the Unabomber produced his 35,000-word "manifesto", excoriating the industrial revolution and modern science, David had a sinking feeling. The tone was chillingly similar to some of the more hate-filled letters he had received from Ted, and there was one phrase in particular he recognised: "Cool-headed logicians."

The recognition that his brother might be the Unabomber sent Kaczynski into a tailspin. "It was a feeling of being trapped – trapped in this brother relationship, trapped in this dilemma in which people's lives were at stake either way. One way, if we did nothing, another bomb might go off and more people might die. The other way, I turned Ted in and he would be executed."

Weeks of agonising followed. His mother's exhortation – never abandon Ted! – rang in his ears, but ultimately the decision was simple. He could not stand idly by and watch more people die. He went to the FBI.

His brother's life was now at stake. Though the authorities assured Kaczynski they would not seek the death penalty, they reneged on the promise. The threatened capital punishment was only dropped after Ted was diagnosed with schizophrenia and pleaded guilty to all charges.

For Kaczynski, the years since Ted was sentenced to life imprisonment in 1996 have been like a prolonged discourse in what it is to be a brother. There has been plenty of time to reflect on what happened, on what Ted did. He doesn't feel guilt so much as regret. "That time I sacked him, could I have been less angry, tried a different approach? Could I have been more understanding, a better brother?"

Kaczynski is still trapped in the definition of being the Unabomber's brother. He now devotes his working life to campaigning against death row, inspired by his sense of betrayal by the federal prosecutors. As head of New Yorkers Against the Death Penalty, he seeks to build bridges between victims of violence and the relatives of the perpetrators. His closest friend is Gary Wright, a computer store owner from Utah who had more than 200 pieces of shrapnel lodged in his body from one of Ted's bombs.

Kaczynski has no idea how Ted is doing in his cage in a high security prison in Colorado. He never replies to letters, and the prison authorities will not say how, or even if, he is being treated for his mental illness. Kaczynski thinks often of that rabbit. "Where Ted is, in some senses, is his worst nightmare. Totally under other people's control, enclosed, cut off from the sky and the wilderness."

Sometimes Kaczynski will be driving down the road from his home in upstate New York and, glancing in the rear-view mirror, he'll see Ted driving the car behind. A moment later, he'll realise it's just another man with a beard. He remembers how the Unabomber once sent a bomb to an airline executive. It was concealed in a hollowed-out copy of a book. Its title: Ice Brothers.

Brothers: 26 Stories of Love and Rivalry, edited by Andrew Blauner and with a foreword by Frank McCourt, is published in the US by Jossey-Bass books.