"If there is one thing sure in the world, it is certainly this: that it will not happen to us a second time."

- Primo Levi, 1958 Survival in Auschwitz

"It happened, therefore it can happen again: this is the core of what we have to say. It can happen, and it can happen everywhere."

- Primo Levi, 1986 The Drowned and the Saved

Philip Gourevitch came across these two quotes by accident. There is an idea, he says, that the Holocaust was so unique it could never happen again, that nothing could ever quite be the same. "That's true," he says, "it is unique, but so is Rwanda." The quotes from Jewish writer and Holocaust survivor Primo Levi, 28 years apart, illustrate both points.

In 1994 as a freelance writer for the New Yorker magazine, Philip Gourevitch was writing a story on the Holocaust Museum in Washington when news broke of the ongoing genocide in Rwanda. "I was standing in line waiting to get in, and on the front page of the Washington Post I was looking at the Rusumo Falls" - over which thousands of bodies cascaded out of Rwanda into Tanzania during the genocide.

His Jewishness, he says, certainly emphasised "the juxtaposition of the very safe rhetoric of `never again' with what was happening". The rhetoric is safe, he said, "because it's not very hard to pluck up the courage to oppose the Nazi extermination of the Jews today". While saying his Jewishness made him "more alert and more perplexed" by what was happening in Rwanda, he does not make simplistic comparisons. "Rwanda is a completely different story than what happened to the Jews," he says.

However, he notes some parallels. His parents and grandparents arrived in the United States as refugees from Nazism. They had stories similar to those of the Rwandan survivors spoken to by Gourevitz - "of being hunted from here to there because they were born as a this and not a that, or because they had chosen to resist the hunters in the service of an opposing political idea."

As children, his parents separately came to the United States as refugees from Europe in 1940. His father teaches at New York University, having retired from Wesleyian University, where he was a professor of philosophy. His mother is a painter. Now aged 37, Philip Gourevitch grew up in Central Connecticut in rural New England but has lived in New York city all his adult life.

From his mid-teens he wanted to be a writer ("I never really considered being anything else") and says he realised at age 20 that "you have to write to be a writer".

So he took three years out from university, where he was studying "this and that in the liberal arts", to write. "I thought I was writing a novel but nothing came of it." He had half a dozen short stories published in US journals.

Then the publishers of The Forward, the once socialist daily paper in the Yiddish language whose readership had dwindled dramatically, decided to revive it by bringing it out in English. Gourevitch wrote a bookreview for the first issue and having suggested that he write more, was asked instead to become New York bureau chief.

He had never read much non-fiction nor thought about doing it much. But he found journalism ("commuting by subway to some newsworthy drama, then coming back to the office and banging it out") very exciting. During three years with The Forward he had begun to develop his journalism into longer magazine articles. Granta sent him on assignment to South East Asia to write about Vietnamese boat people, and he then began working for the New Yorker.

Media coverage of what was happening in Rwanda seemed grossly inadequate to him. A survey of US newspapers by an American academic in the first month of the genocide showed not one newspaper quoted a Rwandan. "It was a kind of faceless, nameless people that had endured a seemingly indescribable calamity. It was being described as anarchy or chaos, yet we were being told that 800,000 people had been killed in 100 days. This did not seem either anarchic or chaotic to me. It requires mass organisation to create that kind of mass destruction."

His attraction to the story led him to go to Rwanda a year later on a three-month assignment to write a 15,000-word feature for the New Yorker. Having done that, however, he felt there was much more he wanted to know. He went six times in all over a 21/2-year period. "This thing got right inside me. In the process of writing I started to see the germs of many stories and thought I'd like to go meet these three people this guy mentioned, or whatever. I'd like to go back.

"I loved it," he says frankly. "It's a complicated story. To simplify it is to render it incomprehensible rather than make it more acceptable. The frustration of writing the third or fourth magazine story was that I had to set the whole thing up each time, explain who everybody was again. The thing about the book is suddenly, 50 or 60 pages in, I felt like the reader would have enough points of reference he could move to a new layer. Small things become significant because they have a context."

His writing rarely enters the common journalistic mode of directly telling readers how the writer feels. Rather, he describes facts and events and retells the testimony of others. "Rather than say `here's the way it made me feel', I prefer to say `feel it, experience it, go through it, think it'." The effect is powerful. Dispassionate, detailed descriptions of piles of corpses of people hacked to death, of how exactly they died, of notes they wrote seeking help before they died, hit the reader with a shock much more severe than an account by the author of how moved/sickened/upset he felt.

"At least fifty mostly decomposed cadavers covered the floor, wadded in clothing, their belongings strewn about and smashed. Macheted skulls had rolled here and there.

"The dead looked like pictures of the dead. They did not smell. They did not buzz with flies. They had been killed thirteen months earlier, and they hadn't been moved. Skin stuck here and there over the bones, many of which lay scattered away from the bodies, dismembered by the killers, or by scavengers - birds, dogs, bugs. The more complete figures looked a lot like people, which they were once."

Such description is still not enough to convey the enormity of what happened. To try to fill in the pieces missing between the facts, Gourevitch brings his imagination to bear. What was it like, he wonders? What was it like at the time, while it was happening?

"A few weeks earlier in Bukavu, Zaire, in the giant market of a refugee camp that was home to many Rwandan Hutu militiamen, I had watched a man butchering a cow with a machete. He was quite expert at his work, taking big, precise strokes that made a sharp hacking noise. The rallying cry to the killers during the genocide was `Do your work!' And I saw that it was work, this butchery; hard work. It took many hacks - two, three, four, five hard hacks - to chop through the cow's leg. How many hacks to dismember a person?" He had to work to keep his emotions out of his writing. "Your imagination is being taxed and your soul is being tugged. You are having this intense emotional and humane response and at the same time you are trying to comprehend, you are trying to get it, you are asking how could . . . ? How could . . . ?

"It's not very interesting to get upset. It seems to me lazy just to be emotional. I wasn't threatened, it's just distressing to see people's distress."

His aim was to keep following that "how" question: how could huge swathes of the Hutu population become killers together and kill 800,000 Tutsis - mainly using hand-held implements - in 100 days?

The book therefore goes beyond the intensity of attempting to chronicle and imagine and convey what happened. In search of the answer to the "why" question, he took on a substantial amount of historical research. Much of this, he maintains, was "unbelievably boring", but he spent a month at it, full time, to give him the background he needed to deconstruct local versions of history.

"Rwandans have a tendency to sit down and give you a history lecture and you can tell their politics from the content of their history lecture," he says, noting a phenomenon not unfamiliar in Ireland. "I can't read enough French for pleasure but can read it for work. I dug up a bunch of French sources and books and dug around, and considered that part of the reporting work." Nor is the book simply about the dead and why they are dead. It is populated by the living - the survivors, the killers and those who saw their relatives killed. Fascinating characters are introduced from time to time - the man who killed his neighbours, the neighbour who survived and now lives up the road from him, the heroic hotel manager who saved hundreds, partly through ensuring Hutu leaders had all the alcohol they required.

He rejects the "ancient ethnic hatreds" explanation trotted out by superior-feeling diplomats both to explain violence in places from Rwanda to former Yugoslavia and to explain why other countries did nothing to prevent it. "I just didn't accept intuitively the idea that something like this can come about spontaneously," he says. Superficial reportage left the impression "that Hutus and Tutsis were a little bit like dogs and cats, genetically programmed to go at it, that this is what they did forever. I wanted to go to try to understand better".

The genocide was not simply a hate crime against the Tutsis, it was a political exercise in unifying Hutus. In the years leading up to the 1994 genocide the Rwandan Hutu regime had agreed, under intense international pressure, that multi-party politics of a sort would be introduced in the near future.

"Those who had had control for 25 years of all Rwanda's political life suddenly faced a peaceful sharing of power, and it was the threat of that that motivated the genocide."

To understand how so many were mobilised to carry out the killing, he says, you have to understand how different Rwandan society was from our societies. Rwandan society was tightly organised and intensively administered. "The better model for what happened is not anarchy but the really well run totalitarian state, where one moves in step, where conformity no longer seems like something one meditates about. There are no other voices, they are simply eliminated, you have a largely illiterate population listening to a single radio station, as are their leaders. Their leaders are only talking to the leaders immediately above them; there is a perfect pecking order, and so that really means that you have got a system where if the guy at the top jerks the string the ripple effect goes straight down, the command goes straight down and if anything in the way obstructs it you can identify where the hitch is and remove the hitch."

ALTHOUGH it is obvious, it is nevertheless striking to find that killers look the same as everyone else. Gourevitch talked to a number of them after the genocide in his search for the "why" answer.

"Those who killed tend to say they were pressured into it, but it seems there was very little active resistance to that pressure. The resistance would have been worn down over the years and years and years during which massacres became a periodic feature of normal life, in which one saw that those who killed were not punished but rewarded, and that was crucial. The extent to which civic rewards - honour, money, spoils - were built into it were crucial. It wasn't seen as a crime to kill during the genocide. It was in a sense to enact the law. It was a world turned on its head."

The scale of what happened is still on the border of comprehension. The 3,000 killed in the 25 years of the Northern Ireland conflict is equivalent to a third of a day's work in Rwanda during the genocide. Could it have happened here? Might we all be capable of doing it?

Comfortingly, Gourevitch thinks it highly unlikely. "In some way it is still understood in Ireland or the United States that this is murder, it's a crime: this is not civic service; this is not without consequences. In America, if they went on the radio and said the time has come to kill your neighbour, I think most people would say `bullshit' and change the channel, because there is a channel to change to."

There are too many political alternatives, too much pluralism for this particular genocide to happen in more diverse societies, he thinks. The Hutu killers killed those who would not join in the mass killing. "The IRA was not about to start killing unco-operative potential supporters to make everyone realise they had no choice but to support them. To get to the stage where that is even an option takes an incredible amount of political organisation over a long period of time, a complete reduction of political opposition."

But he adds that it is still impossible to fully analyse and explain what happened. "I can observe and explain the mechanisms and describe how it happened and describe the context in which it happened, but I think even Rwandans still feel that in some way it remains a mystery to them, not just among those targeted for death but even among those who killed."

Could it have it been avoided? Would there have been different outcomes for, say, Rwanda and Yugoslavia had they been run by Mandela and Adenauer rather than Habyarimana and Milosevic? Yes, he says. Tito was hardly a Mandela, yet few talked of the necessity of ethnic cleansing while he was there. "Kagame's [Rwanda's present leader] line that people can be made bad and taught to be good struck me enormously and helped me understand what happened there and what could happen. It's a terribly scary thought and a terribly hopeful thought at the same time."

Everyone gets around to asking Philip Gourevitch if there is any hope for Rwanda. He says there is. "One of the amazing things is just how deep is the human determination to go on. Your writer Beckett got at it and he certainly wasn't wrong. What's the alternative? To hold your head in your hands and weep in the rubble for years? People don't do that. People go for it. People live. I was amazed at how many survivors remarried. They had lost their families and they were starting to reconstruct new families."

He tells a simultaneously uplifting and horrible story at the end of the book. Twice in April 1997, three years after the genocide, a party of genocidaires - as the killers are known by all in Rwanda - went to boarding schools to kill Tutsi students. On each occasion they roused the students - teenage girls - from their sleep and ordered them to separate themselves, Hutus from Tutsis. In both cases the students refused, saying they were simply Rwandans. A total of 34 were beaten and shot to death. The stories were reported on Rwandan television.

"If you want to find hope," he says, "it is going to look like this. You are going to have to find heroism among Hutus of conscience. It's easy for Tutsis to say we should all be Rwandans, not Hutu and Tutsi, because they are a minority and minority elites are always for assimilation and meritocracy. The courage to resist Hutu power has to come from Hutus."

We Wish to Inform you that Tomorrow we will be Killed with our Families, by Philip Gourevitch, is published by Picador at £16.99.