Often called the Garden of Eden by western travellers, the foothills of the mighty Indian Himalayas have it all – snow-clad peaks, magnificent remote valleys and gorges, life-giving rivers and endless banks of wild flowers. There are worse places to spend the last week of your life.

The last thing on Susan East's mind was death. Like many young westerners in the late 60s and early 70s, she had been drawn to the hippy trail. It promised adventure and a new life, far away from Canterbury, where she had very recently given birth to a baby, given up for adoption.

I was that baby.

It is now understood that giving up a child for adoption is deeply traumatic for the mother but in those days there was little compassion or understanding. Instead there was a burden of shame attached to teenage pregnancy outside marriage. Susan's vision was fixed on India, home of the eastern philosophies that fuelled the hippy way of life, far from the traditional restrictions of society.

Her parents, kindly but conventional, did not want their fifth and youngest child to head off alone on such a dangerous journey. But two things worked in Susan's favour. First, the government reduced the age of majority from 21 to 18, and second, her grandmother gave her some money. Reluctantly, her parents waved her off.

Ten months after I was born in October 1971, Susan travelled first to Istanbul, the point at which all roads from Europe converged, continuing across Iran, which was then a secular country run by the shah, and on into Afghanistan, where foreigners were welcomed and, not insignificantly, the hashish was plentiful.

The wandering pot-smoking westerners always preferred the term "freaks" to "hippies" and must indeed have looked like freaks to the Afghans, who were largely unaccustomed to tourists of any sort. But people were hospitable and Susan loved Kabul.

The following May, Susan arrived in Manali, at the north of the Kullu Valley in India. She fell in love with the small town, famously beautiful, nestling under mountains that carved a sharp outline against the blue sky. Come winter, she planned to head south to the beaches of Goa.

But a few weeks later, Susan was dead.

There was no funeral, at least not for her family, because the cremation was quick, as was the custom in India. Worse, there was no explanation for Susan's death. In fact, the whole episode remained a mystery for almost 30 years – until I discovered that she had almost certainly been murdered.

To me, growing up, Susan was always a mysterious figure. In a middle-class family, there was only one thing for it, other than abortion, when a teenager got pregnant – to send the girl away to a mother-and-baby home, where a married couple would later collect the baby and the girl would have a chance to put the whole ghastly experience behind her. In my mother's case, not even her siblings knew about it.

For me, this meant a new life in a new family from when I was six weeks old. It was a life so cut off from the original one that even when Susan died the following summer, my family were not told. Indeed, all I knew about my adoption when I was growing up were three bare facts. First, my mother was 17 when I was born; second, she had been in a relationship with my father for at least a year; and third, she had a sibling with a musical talent.

It was the last fact that bothered me most as it meant weekly piano lessons with an old woman up the road who had bent fingers and a sharp tongue. My mum was only trying to honour the one thing she knew about my genes but unfortunately, the musical gene had passed me by.

Later on, the first two facts got me thinking. Did having me so young meant she'd forgotten about me? Did she look like me? What happened to my father?

My adoptive parents were, and are, my family. We named our daughter Lucy after my mum and when my dad died four years ago, the fact that he wasn't genetically connected never entered into how I felt. But by the time I was 18 – the age at which an adopted person can legally undertake a search for birth relatives in England, Wales and Northern Ireland – I felt a strong need to know my roots. It was 1989 and Susan would have been just 36, so while I was (sort of) prepared for her to turn me away when we met, the last thing I expected was that she would be dead.

Kate Hilpern with her mum, Lucy, on the day of her adoption.

Susan's parents, whom I met two days after discovering this news, welcomed me with warmth, exuberance and Canadian accents. They still lived in a 1950s house in Canterbury, where Susan had grown up after their move from Toronto when she was five. They revelled in telling me about her childhood. They invited me to call them Grandma and Grandpa and, best of all, showed me photographs. Even now, I find it hard to explain how astounding it is to see yourself reflected in someone else's features for the first time. At last, someone I looked like.

In retrospect, I suspect that day was far more overwhelming for Susan's parents than it was for me – they had lost their daughter at 18 and gained a granddaughter at 19, the very image of her.

I saw them often and they opened their hearts, admitting their embarrassment in allowing the adoption to ever take place. They told me that Susan had quickly changed her mind about it and got close to a nervous breakdown, especially when the authorities wouldn't even let her have a photograph of me (although they did give in) and how – against her parents' better judgment – she headed off to India.

Susan kept her promise to write regularly. Indeed, my grandparents still had the stack of Basildon Bond airmail letters, tied up neatly with string. I pored over the words, so full of description and promise, in writing so small that I had to hold a ruler under each line. The last letter was sent in May 1972. "I'm so grateful to you for giving me a chance to see some of India," she signed off.

Susan East, Kate's birth mother.

The next letter was not from Susan, but some people she had met in India. They told my grandparents they were sorry to hear of Susan's death and included a couple of her personal things. That was how they found out their daughter was dead.

"She died of a broken heart," my grandmother used to say, shaking her head sadly. Pressed (tentatively, as I did not want to upset her) she said it was a tsetse fly. "Yes, that's it, it was sleeping sickness."

Knowing the tsetse fly doesn't live in India, I wondered if at last the truth might be coming out when she confided in me one day, "They tried to make out it was suicide, you know."

It transpired that Susan's brother had got life insurance for the whole family, but when Susan died, the company wouldn't pay, claiming she had killed herself. But they withdrew the claim and paid up. Perhaps, I concluded, my grandparents just didn't want to, or weren't able to, face thinking about what happened to her.

Two days after meeting my grandparents, I met my birth father, Mark. We formed a close bond right away. He was more curious. He had loved Susan. And he was certainly more clued up about the casualties of the hippy trail. But he was none the wiser about just how she had died. Nobody would ever know for sure, he said.

Despite my questions, though, it wasn't really how she'd died, but the fact that she was dead at all – that was the sticking point. I felt a huge wrench that I'd never know this woman who had been alive in my mind for so many years and who had given me life.

So I tried to get to know her, by talking to family and friends, anyone who had known her. Her friends described her as an intelligent girl with a wild nature, while her siblings knew her as conformist and sensible. Even the content of letters, which I'm so lucky to have, don't quite tally up. Four days after I was born, she wrote to my father with passion and sadness, then mentions nothing about either me or him in the letters posted on her travels.

Still, it helped, as did my trip to Susan's birth city of Toronto, when I was 21. I'd gone to meet my Canadian cousins and decided to spend a year there. One day, it occurred to me to ask the Canadian embassy if they had any details about Susan's death. To my amazement, they gave me not just her death certificate but the post-mortem results.

There in black handwriting, one stark word jumped out and shocked me to the core: Poisoning. Friends were sure it was probably food poisoning. But the documents said she was found unconscious alone by a river bank without her passport or money (neither of which were ever found). Something wasn't right.

Kate with her birth father, Mark. They went to India to find out what had happened to Susan.

"We should go to India," announced Mark after I'd returned to England. It would be a kind of pilgrimage for Susan, he said, and a tangible part of my bereavement. Besides, it was my only hope of discovering the truth about what really happened.

So, in 1999, 27 years after her death, we flew to Delhi and retraced Susan's steps as backpackers ourselves, eventually reaching Manali. Sure enough, the mountain scenery was striking, but disappointingly crammed with concrete hotels. In Susan's day, there had been a handful of guesthouses; now there were more than 650. At least the old town of Manali remained a peaceful sanctuary, with fewer dwellings and the clearest, cleanest air imaginable. The extreme poverty was profound, however, and with cannabis growing plentifully, the westerners spending all day high on pot was not a pretty sight either.

Susan died at the busy Kullu district hospital, where the staff working in the crumbling buildings took time out for us when it was quieter. When the chief doctor read our documents, she smiled sympathetically and explained, clearly not for the first time, that, yes, the poison was the type that is given to people with the aim of knocking them unconscious. It's usually put in pot, she said, and while some people survive, others don't. The objective is usually robbery. There was no doubt in her voice – she had seen such documents and heard such tales many times, and her peers confirmed that Susan's story was far from unique.

Stunned, I listened to how Kullu, originally named Kulanthapitha, which literally means "the end of the habitable world", has witnessed countless westerners killed or gone missing.

"Some people describe this area as lawless," said the chief doctor at Manali's smaller hospital where my mother was initially taken. "In some villages, there is a complete lack of principles. What's more, you will never find a local admitting to witnessing any kind of injury or death for he knows someone might punish him for it. So justice is rarely achieved or even sought."

The following night, we discovered this for ourselves as a 32-year-old Scot, who had arrived on the bus from Delhi that morning and whom we had seen in a bar that evening, was found dead on the roadside – nobody claimed to know anything about it. We thought of his poor family.

Things are worse now. Smoking hashish, at least among adults, has been part of the traditional culture for as long as anyone can remember. Young Kullu boys sniffing bonded resin and locals hooked so badly on heroin that they will never find work again has nothing to do with tradition. Nowadays, we learned, the stunning hills and gorges hide a community of dropouts and drug dealers far from the reach of embassies or even the local police, with the inevitable drug wars, drug-related crime and a huge number of deaths.

Indeed, four years after our trip, an article in this newspaper reported on the many British backpackers who come to Kullu in search of peace, tranquillity and the world's strongest hash, but whose battered bodies are washed up in the river or whose skeletons are found in sleeping bags. Murder, corruption and drug-trafficking flourish in this valley, the writer concluded.

Susan, not long after Kate was born and before she left England to travel.

On the last day of our trip, Mark and I visited the Bhuth Natha Temple on the banks of the river, where Susan was cremated. We knew the visit would be hard, but hoped to lay her spirit to rest. But there was nothing remotely dignified or even secluded about the stark metal grate, the exact size of a human body and riddled with rust. We scuttled off as quickly as we'd arrived, bewildered and tearful. But when we returned a few hours later, somehow the fatalistic, spiritual nature of India seemed to do its job and it wound up feeling a cathartic, if not apt, ending to our journey.

Mark and I don't talk much about what we discovered. Life goes on – I have since had a family of my own and he travels a lot – in any case, when I think back to that trip, I remember it more as a summoning up of her spirit, one that enabled us both to finally lay her ghost to rest. I also remember it as a uniquely special time for Mark and me. Perhaps most importantly, when I think of Susan now I like to focus on the life she had, not the death.

But from time to time, I am still haunted by it. I wonder whether the person who poisoned her ever felt a prick of conscience. I wonder if she knew she was dying during the last few days of her life. I wonder what she'd make of the world – and me and my family – now. Of course, the facts would be hard to come to terms with, no matter how she died. But they are all the more painful knowing her death was avoidable and that such deaths are happening more than ever and still leaving families with unanswered questions.