They move, tentatively, on this day of commemoration among desolate, rust-coloured industrial buildings, haunted by what happened within them. Nusreta Sivac places a flower on each space of floor where her dead friends once slept in the quarters for women who "served food and cleaned the walls of the torture rooms, covered with blood" - quarters just across a hallway from the now empty office where she was, like them, serially raped, night after night. And she passes the window from which she watched the slaughter of men on the tarmac below, day in, day out.

Satko Mujagic knows that tarmac well: his two-year-old daughter now plays with a ball on the very spot where he had been too weak to line up for bread because of dysentery, and had to be supported by his father. Later, the child picks a daisy. "You do this where your father lay bleeding," says one of the party. "Being here gives me the feeling of understanding nothing," says Satko. "The violence here was nothing to do with anything, not even war. It is unfathomable."

Nusreta tells the story of a family typical of Omarska and its legacy; one family among the thousands. "It was the night of one of their saints, St Peter," she recalls. "The guards were drunk and set tyres on fire, singing their songs and screaming as they took prisoners out to jump on them and beat them to death. One man, Becir Medunjanin, was being jumped upon, while his wife Sadeta watched from our quarters. She cried out, 'What are they doing to him?' and I tried to calm her lest she lost control and was taken out too. Sadeta was later killed as well. They had two sons; one had already been killed when they shelled the village - Sadeta always said that if she survived Omarska she would find his body to give it a proper burial. The other, Anes, survived Omarska, the only member of the family to live. He came with me just recently to identify Sadeta's body and gave his DNA. 'That is my mother,' he said."

The date of this commemoration of the camp's closure - August 6 - is branded into these people's minds. And I have a stake in all this: for the closure of Omarska followed the day after the putrid afternoon of August 5 1992, on which it had been my accursed honour to find a way into this place, along with a crew from ITN.

We saw little that day, but enough: terrified men emerging from a hangar, in various states of decay - some skeletal, heads shaven - and drilled across a tarmac yard, under the watchful eye of a machine-gun post, into a canteen where they wolfed down watery bean stew like famished dogs, skin folded like parchment over their bones. "I do not want to tell any lies," said one prisoner, "but I cannot tell the truth." And it is strange - traumatic, indeed - to stand again in that now empty canteen; strange to walk that tarmac killing ground.

It is disturbing to wander these dread buildings - where inmates were held and beaten, and whence they were called to their death; buildings forbidden to us that day in 1992, our paths blocked by armed guards and the camp commander, Zjelko Meakic, now awaiting trial in the Hague. Disturbing also to see the so-called Red House, where prisoners' throats were cut.

The feeling is all the more strange when I recognise a man I had met that day, in that same canteen: Sefer Haskic, who is now a joiner in Bolton, revisits the room into which he was crammed. "I was trying to remember the people they killed," he says. "All my friends. They would call out the names, and men would get up, leave us, and never come back. You could hear the screaming, the killing, you could smell burning tyres and dead bodies. Next morning, there would usually be about 30 of them: the yellow truck would arrive so that other prisoners could load them up and go to dig graves. The truck would always come back, but the men who loaded it usually not. I was forever waiting my turn, but it never came - I still can't believe I'm alive." Sefer remembers in particular a night of frenzied ferocity, during which some 150 men were killed, "and the walls were covered with blood".

However, these people have not returned to Omarska only for remembrance; it is also a gesture of defiance. It was intended by the Bosnian Serbs - as has been affirmed at The Hague - that no Muslims (or rather Bosniaks - the secular ethnic term by which they are properly known) should remain on this territory alive; that they should all be deported or killed. But all around us now are the sights and sounds of a once unthinkable return by thousands of Bosniaks to the homes from which they were brutally expelled. They come back under the shadow and insignia of their persecutors, with whom they live cheek by jowl - for this is the so-called "Republika Srpska" granted to the Bosnian Serbs at Dayton in 1995. But they do so all the same.

They return also to the village of Kozarac, the site of a savage attack on May 24 1992. It was emptied of all 25,000 Bosniak inhabitants. Every Muslim house was marked in paint for incineration; the surviving Muslims herded in droves over the mountains at gunpoint. But the place is now home to more than 6,000 Bosniak "returnees", who outnumber the Serbs as they did before, with an additional 15,000 visiting from the scattered diaspora for summer. Once again, minarets - blown apart by the Serbs - nestle, rebuilt, against the hillside.

With much greater difficulty, people return also to the local seat of authority, Prijedor, where the persecutions were planned and whence orders for establishment of the camps, for the killing and mass deportation were given. In Prijedor returnees live under the cold stare of their erstwhile persecutors; but Kozarac is an effervescent, if peculiar, place. As families sit out to enjoy pizza and beer in the warm evening, so they recognise one another: a survivor of Omarska here, of another camp there, a bereaved father here, a widowed mother there. The entire community is a concentration camp survivors' reunion. Everyone here is damaged, but resilient. No life is unaffected by the maelstrom of violence.

If there is a driving force behind the return to Kozarac, it is the quietly composed figure of Sabaduhin Garibovic, who runs the Concentration Camp Survivors' Association. "We are doing this," he says, "to show the Serbs who evicted us that they did not entirely succeed. That we can come back. They never thought they would see it. They cannot fathom what we are doing."

Sabahudin's father survived Omarska, but his brother Armin was among the first to die there, his name called from among 156 men packed into the "garage", a space just five metres by six. There was no water: the men had to drink urine to live. It was so hot that the prisoners smashed an upper window to let in air, for which Armin and another man were murdered. Sabahudin himself is a survivor of Trnopolje, another camp we entered that day in 1992: "I remember them taking out the girls to do what they would with them - six or so each night, including my niece." Trnopolje was the location for the enduring image of the war: the skeletal Fikret Alic and other prisoners behind barbed wire.

"Almost every day I see the people who did this to us," says Sabahudin. "We live separate lives - there is nothing that unifies us with the Serbs. We rely on ourselves and each other to survive." Just before our meeting, a jubilant wedding motorcade passed through town, hooting and waving the old Bosnian wartime flag. In overwhelmingly Serbian Prijedor, it was pelted with bottles and rocks. Two weeks before, a bomb had been thrown at a Bosniak-owned bar in Kozarac; a Serbian former camp guard living near Omarska was beaten up by Bosniaks. There are countless such incidents. "International foundations organise round tables to discuss living together," says Sabahudin, "but it is empty talk, and the reasons are simple: we cannot forgive or forget what happened, and they either deny it happened or say they had to do it - they were obeying orders."

Kozarac's economy depends almost entirely on the diaspora - on Omarska survivors such as Edin Kararic, who now works as a tanker driver based in Watford. Edin has managed to put some money into buying a cafe called Mustang on Kozarac's main drag, managed for him by a fellow survivor. "They drove us out," says Edin, "and we are buying it back. This cafe is my finger stuck up to the Serbs who did not want us here. In fact, that is what those minarets are, on the mosques that no one goes to: fingers stuck up at the Serbs. That is why we must come back to this place - why else would any of us want to, given what happened here?

"Mind you," he adds, pensively, "it's difficult to enjoy yourself in a place where 7,000 people are missing from a population of 25,000."

Emsuda Mujagic was among the first to come back to Kozarac, having been a refugee in Croatia. "I wanted to see in the new millennium at home," she says, "and so I came back on December 31 1999. Our house was one of the first to be destroyed in the shelling, but we rebuilt it slowly. There was literally nothing here. No birds, just snakes and a few Chetniks [slang for Serbs]. I have to stand up to their plan, which was to destroy not just a community but a whole people. That is the wish that has kept me going."

Emsuda is a survivor of Trnopolje, and on the 12th anniversary of our discovery of the camp, she takes me back to what is now a school again, closed for summer. There, sitting on the steps, Esmuda recalls how each night "the guards would just walk by and shoot or beat people while we slept in the open. Or else they would come into the women's and children's quarters with torches and read the names of young girls from a list, some as young as 10, 12 or 13. They would take them to a house where Serbian soldiers from the front would have their way with them. Some of the girls would come back, scarred and tortured - others would not, and we understood they had been tortured to death. One woman was breastfeeding her baby when they took her - she gave the child for safekeeping and came back horribly scarred."

Nusreta, who struggled to come to terms with her ordeal in Omarska, steeled herself to return to Prijedor in July 2002. By way of welcome, she found the word "Omarska" scrawled across her doorway by her new neighbours. "At first I thought I wouldn't be able to bear it," she says. "I used to stay indoors, peeping through the curtains."

There was always a macabre intimacy to Bosnia's war - people knew their torturers and murderers - and the intimacy remains. "A lot of the Omarska guards live in my neighbourhood," says Nusreta. "I see them almost every day. One of them, called Vokic, has his entrance in the next block of flats and we share a bedroom wall. I see the interrogators and even the man who ordered that I be put in Omarska - he's a bank manager and drives a Mercedes. I try to catch his eye, but he turns away. Another has been let out from prison in The Hague - called Kvocka. Last time I looked him in the eye was when he was in the dock and I was a witness. But I often see him on the street, even on the day we went to buy flowers for the burials of five women from Omarska whose bodies had been exhumed. There he was, in the florist buying flowers for his wife. I said to my friend: 'Look, Kvocka is standing behind you. On the day the dead are buried, and thousands more are dead, he walks free.'"

Nusreta, a former judge, returned not to her own apartment but to her brother's. Why? When she emerged alive from Omarska, she explains, she found a former typist from the bench called Ankica living in her flat, and was invited in for coffee. "There I was, like someone gone mad," recalls Nusreta, "straight from Omarska and a guest in my own flat. I sat down on my sofa. Ankica, wearing my clothes, made me coffee in my pot, served in the china my mother left me, and asked me: 'Why are you acting so strange?' She said the apartment suited her, she had always wanted one like this."

Years later, Nusreta returned - as was her right under the Dayton peace plan - to be promised by Ankica that everything would be left in order. "But when I finally evicted her," says Nusreta, "it had all gone. Even the built-in wardrobe. Everything I had inherited from my mother. Even my photographs. It was pure spite, to wipe out my past." Thankfully, Nusreta has a few good friends in Prijedor, notably the only Bosniak doctor in town, Azra, whose elderly father and stepmother had their throats cut when they returned home after surviving Omarska in 1992.

"Sometimes I get a crisis in the night," says Nusreta, "that someone may knock at the door or throw a brick through my window. But I will become happier in accordance with how many of our people come back. My only wish is that by us coming home, the Serbs do not get what they wanted." However, she says by way of conclusion, "I can never again be happy."

One hallmark of the aftermath of Bosnia's war is an almost complete lack of reckoning on the part of the Bosnian Serbs. Only one defendant - the former Bosnian Serb joint-president herself, Biljana Plavsic - has pleaded guilty at The Hague towhat happened, and appealed for reconciliation. But around Omarska, the returnees' narrative falls down a black hole in the perpetrators' memory. "There was no camp here," security guards at the entrance to Omarska mine told us. "It was all lies, Muslim lies, and forgery by the journalists."

"There is no remorse," says Nusreta. "No one has apologised or even admitted what happened. They say they know nothing about the camps. There are 145 mass graves and hundreds of individual graves in this region, and we invite the local authorities to our commemorations, but they never come." "Even now," says the Bosniak political leader in Prijedor, Muharem Murselovic, "the Serbs will not accept that anything happened. I am always in a dilemma - are they crazy, or are they pretending to be crazy? I think it is because they were all so deeply involved in what was happening that they cannot come forward and admit it."

"Every time I see a Serb who is extremist," says Sabahudin, "I remind him of what happened in front of their eyes. In such a way as I hope might change his viewpoint. He has to understand that if this country is to survive, they have to change their mind. Any future together is conditional upon them admitting what they did, and apologising for it."

The security guards from the all-Serbian village of Omarska signal that it is time for the commemorative procession to leave the camp. But as we leave, there remains one urgent question, one burning uncertainty.

Crucial to the reckoning of which Sabahudin speaks is the matter of the future of the site of camp Omarska. There is nothing to mark what happened here - the horrors are officially buried, hidden, denied. The Serbian local authorities are enthusiastically pursuing a plan to sell off the mine to overseas investors, which could result in the concealment of a mass grave, a monument to barbarity and suffering. The killing ground could become a car park. The physical memory of this evil but sacred ground could be obliterated.

Bosniak expectations are modest, and quite possibly doomed. "We would be pleased," says Sabahudin, "if there could just be some kind of memorial, maybe that the White House might be fenced off. We just want something to ensure that the memory is preserved, and in the smallest way to awaken the conscience of the Serbs. That is the really important thing. Because if we don't awaken that conscience, we might as well forget everything. And that would be the saddest thing of all - to forget what happened and what could happen again tomorrow. Yes, tomorrow."