They are the unquiet dead. Laid out in rows inside a former industrial building on the edge of the Bosnian town of Sanski Most. Some of the skeletons are almost complete, others just a pelvic bone and some assorted ribs, arranged as though to await the arrival of more.

This place was used to process wood before Bosnia’s war of the early 1990s, and now it processes – it endeavours to assemble – the dead. The remains are laid out on raised trays, and at the foot of each lies possessions found with the body when it was exhumed, invariably from a mass grave. So to walk through this hall of death is also to walk through these people’s lives and last moments. A pair of trainers here, a checked shirt there, a watch or wallet. What made this person choose a yellow sweater rather than another on a market rail, and chance to be wearing it when taken out to be murdered? Why striped socks beneath this half-assembly of bones, plain ones to accompany the next? Who were these people?

In this building, run by the Krajina Identification Project, there is diligent purpose. These dead people had been missing for 24 years, along with tens of thousands of others, while their families – survivors of the hurricane of violence that blew through this corner of Europe – searched, wondered, feared the worst.

This facility is one in a chain that seeks to answer that question, the work of which is the most remarkable entwinement of science, human rights and justice in the world today. The task is to locate and exhume the 40,000 people who went missing after the western Balkan wars – the worst carnage to blight Europe since the Third Reich – then to assemble their remains insofar as they can be found, identify them, give them names, and return these dead back to the living for burial. It is scientific work at its most committed and advanced, helping to meet humankind’s most primal need: to bury or in some way ritualise the remains of our dead.

Beside a rough road that climbs a remote mountainside, between the towns of Prijedor and Sanski Most, lies the house that Zijad Bačić has rebuilt in the hamlet of Čarakovo, from the ashes to which it was charred in 1992. Next to the house, where he now plays football with his son, Adin, is a modest marble monument, on which are carved the names of 38 people, many of them members of Zijad’s extended family. Some of them were killed on the night of 25 July 1992; others vanished. Zijad was 15 years old on the night that – after his father and most other men had been taken away to concentration camps – Serbian death squads came back to “mop up” the women and children.

I watched my mother hit first and fall down, then my brother and sister – and I ran behind a bush to hide

“We were at home,” he recalls, “when we heard the soldiers’ voices. ‘Come on out! Come on out!’ My mother gestured to us: we must. As soon as they went out, and the other families around us, machine guns began firing. I recognised one of the men, the others wore balaclavas. They were about five years older than me. I watched my mother hit first and fall down, then my brother and sister – and I ran behind a bush to hide. I stayed there until they had finished shooting and shouting – I recognised another of the balaclava men from his voice; they came from just down in the valley, they were neighbours.

“My mother, and my brother and sister were dead. But I survived. I was deported on the convoys, and went to a refugee camp in Germany. And I never thought I’d come back here, but I couldn’t sleep without knowing what happened. Where were they? I had to find my missing father, all my uncles, and to find where they had buried my mother, younger brother and sister.”

There have been seven arrests in connection with the extermination of the Čarakovo villagers. Two of the accused have been released on bail under house arrest, and Zijad thinks they are the men he recognised on the night of the massacre. “That’s one of their houses right there,” he says, pointing towards a white building on the valley floor. “We’re hoping that these trials will reveal where my family is. I’m going to testify. Even though we’re surrounded by them, I have no fear of anyone or anything any more. My only need in life is to find those I have lost.”

Where Zijad’s mountain lane meets a tarmac byway, there stands a little shop kept by Zijad’s uncle, Fikret Bačić, who takes my notebook and writes a list of the names of his extended family who went missing during the last week of July 1992. It takes him a long time; there are 29 of them, including his mother Sehriša, his wife Ninka, son Nermin, who was 12, daughter Nermina, who was six, four brothers including Zijad’s father, three sisters, several aunts, uncles and cousins. Of the 29, 19 are children; the youngest was two.

Facebook Twitter Pinterest Prijedor, where some of the worst atrocities of the Bosnian war took place. Photograph: Andrew Testa/Panos

“They were taken and killed by the viaduct, just there, by the main road,” Fikret says, pointing to a railway bridge beneath which we’d just driven. “For years, I’ve had no idea where they were buried.

Fikret was working in Germany when it happened. The first thing he did was to tour refugee camps across Europe: Holland, France, Austria, Croatia. “I hardly knew what I was doing, like a wild hunter-dog. But nothing. So there was only one thing to do: come back, and I never thought I’d do that, back to the destroyed house. But I did, in 1998, only to start looking, for I had nothing else to do with my life but find the bodies and the people who did this. I asked a Serb, who had been the best man at my wedding, raised by my grandmother: where are they? There are not many people I can ask, I pleaded with him, I just need to know who did this, and where they are buried. He just said: ‘I don’t know, I was not there.’ I could tell he was lying. I went to the police in Prijedor, but everyone knew it had been the police who organised the hiding of the bodies. Then the other, but he had died. Then I realised the only thing to do was to start digging.

“I dug everywhere. I helped wherever there was a dig.” Fikret went to the mass grave found at the village of Kevljani in 1999, next to a concentration camp established by the Bosnian Serbs at Omarska. There was no sign of Fikret’s family.

Then, in 2004, work began at a second mass grave, also near Omarska, at Tomašica. Fikret was there, digging, but the bodies found there still did not include his family. “None of us knew,” he says, that “we were only 100 metres from the biggest mass grave of them all”.

Facebook Twitter Pinterest A marble monument with the names of 38 people who were taken from the small hamlet of Carakovo on the night of 25 July 1992 and killed. Many remain missing. Photograph: Andrew Testa/Panos

We sit in Fikret’s yard, roses climbing the fence, neighbours passing by, saluting him. He sips a glass of Nektar beer, a Bosnian Serb brew. “The pain doesn’t go away, it gets worse, stronger, the longer it lasts,” he says. “I went to the state court, and an American prosecutor showed an interest for a while, but then said he had to leave and take another job. After a while, I gave up, I couldn’t go on any more.”

Then, in 2013, there was a bigger, macabre discovery at Tomašica: hundreds more bodies, buried, hidden – but now revealed. Fikret Bačić was there moments after the first earth was broken. “First, they started to find my neighbours, the Tatarević brothers, up the lane there. Then a cousin of mine. And then my brother Refik – no documents, but the DNA matched.

“It’s hard to say what I felt. It was like someone who belongs to me coming back from 10 metres deep. He’s my family. And then another body comes out, and it’s not one of mine, and you feel so bad. I did that for three months, until the last body was found and either identified or not. Now we must wait for another grave to contain the women and children: two were found at Tomašica, but still 17 kids are missing, aged two to 16. You know, I can’t believe I’m saying this; it leaves a bad taste in my mouth, and in this lovely evening, for me to have to tell you that all this is true, that this is what people do to each other.”

Facebook Twitter Pinterest Zijad Bačić has rebuilt the family home in Čarakovo. In 1992 it was burned dowm Photograph: Andrew Testa/Panos

The driving force behind this search for the missing dead is the International Commission on Missing Persons (ICMP), founded in 1996 on the basis of an initiative by President Bill Clinton. ICMP arrived to urge the location and identification of 30,000 people in Bosnia (and 10,000 more across the region), “missing” in mass graves. Most have been found and their remains returned to their families – but 8,000 in Bosnia are still missing.

ICMP has been working on the site of World Trade Center in New York, advising on how to identify those killed on 9/11; it has been in Guatemala and El Salvador, searching for the missing from US-led “dirty wars” of the 1980s; and South-east Asia, identifying victims of the tsunami in 2004. The organisation, says its director-general, Kathryne Bomberger, is thinking about future work that might include searching for the victims of not only conflict but natural disaster, enforced disappearance, human trafficking – and migration. Detailed country programmes have been devised for Iraq, Sri Lanka, Colombia, Albania and Ukraine. It is even preparing to address the as yet unquantified missing of the Syrian conflict – and the 10,000 missing children from the migration crisis in Europe.

“We do this from a premise that all missing persons have rights, the same rights,” says Bomberger. “But in addition to the humanitarian principles, this is also about the rule of law – it is an obligation of states under international law to find missing persons.”

The money this work will require is minuscule in comparison to corporate or even aid budgets – and yet, “this is an increasingly mean world when it comes to funding,” says Bomberger, and the missing are easily overlooked.

“The world out there is one big NN mass grave,” says Ian Hanson, the man who broke the very first ground here in Bosnia in search of the 8,100 victims of the Srebrenica massacre in July 1995. “NN” is the abbreviation used on graves; it stands for “No Name”.

Over the summer of 1991, the break-up of Yugoslavia began to turn bloody, first in Slovenia, then Croatia, then Bosnia, as Yugoslav republics sought independence and Slobodan Milošević’s government in Belgrade sought to establish borders for a “Greater Serbia”, which spread into both Croatia and Bosnia and entailed the elimination, through death or deportation, of every non-Serb in the territory.

In Bosnia, a savage pogrom was unleashed in spring 1992, mainly at first against Slavic Muslims in the east and against Bosniak Muslims and Catholic Croats in the north-west Krajina; the Bosnian capital of Sarajevo was subjected to relentless siege and the stillborn republic torn apart.

I reported on this war, and in August 1992 discovered concentration camps, established by the Serbs for Muslim and Croat inmates, near the town of Prijedor in Krajina. The carnage dragged on until soon after the Srebrenica massacre three bloody years later. I have kept in touch with the survivors of, and those bereaved by, those camps, and come to understand how the outrage of “disappearance” inflicts an immeasurable pain on those who remain. I return to Bosnia every year for commemorations at the camp, and hear how, in so many ways, those words “missing” and “disappeared” are crueller than “dead”; they leave the mothers, fathers and family without so much as an interment, a grave to visit, an account of what happened and why.

When investigators from the war crimes tribunal in the Hague first arrived in 1996 to build a case against the perpetrators of the Srebrenica massacre, their first preposterous task was to search for the evidence: its victims, 8,100 murdered men and boys ploughed into the ground.

They were led by a French investigator called Jean-René Ruez, an anthropologist called Richard Wright, who had worked on second world war graves in Ukraine, and a former archaeologist of ancient and medieval London, Ian Hanson – who is now deputy director of forensic sciences, anthropology and archaeology at ICMP.

Work at Srebrenica began on what were thought to be the five mass burial sites – each containing many separate graves – in which the dead had been buried and left hidden. Then a macabre truth emerged: testing showed that body parts from what became called the primary graves had been moved to secondary ones, to hide evidence. Sometimes, they had even been disinterred and reinterred again, into tertiary graves.

This had two implications: first, that more than a million-and-a-half bones and body parts from 8,100 people were scattered across innumerable sites; and second, that the few byways of rural eastern Bosnia had for weeks, months, been heaving with trucks carrying the rotting, stinking remains of these people – some 3.2 tonnes of “putrefactive material”. Yet no one said a thing.

“We call this ‘grave-robbing’,” says Hanson. The Serbs had arranged secondary graves “to be located in places where there had been armed confrontations, so that they could plead that massacre victims were killed in combat. It was all very carefully worked out.”

The search for the missing was initially regarded as a humanitarian affair. But the war crimes tribunal’s motivation was prosecutorial. When ICMP arrived in 1996, it sought to find the hidden dead both for human reasons, and to find hard evidence to establish what had happened and uphold the rule of law. The victims clearly approve: of the relatives of the missing who gave blood samples in pursuit of a DNA match, 90% agreed to allow any results to be used in evidence at trial.

Facebook Twitter Pinterest Fikret Bačić at home in Carakovo: 29 members of his family were taken by Bosnian Serb forces and killed in July 1992. He is still searching for the remains of most of them. Photograph: Andrew Testa/Panos

From the outset, the process of finding and identifying the dead was hampered by a toxic atmosphere of denial, non-cooperation and sectarian structures that dealt with their own side’s losses and no one else’s – markedly among the main perpetrators responsible for more than 80% of the missing, the Bosnian Serbs.

“When we first arrived,” says Hanson, who was then with the war crimes tribunal, “the people with the information we needed were not nice guys; they were guys with guns not wanting us to do what we had come to do. We used to go into the police stations that were supposed to be helping us, and see pictures of ourselves on the wall: ‘Do Not Cooperate with These People’!”

To that end, ICMP stepped in, not just to help look for bodies and nurture the expertise to do so, but to “assist” the Bosnian government in turning a haphazard, sectarian search into a systematised, centralised operation. Hanson uses the word “assist” but moves his flattened palms against thin air making as if to push it.

The initial search focused mainly on Srebrenica. Identification was at first done using classical anthropological methods: identifying possessions, dental treatment, clothing and so on. But from 2000, ICMP began using DNA samples from blood given by relatives of the dead, matching them with those gleaned from samples of excavated bone. This was the second revolution in forensic anthropology and the figures speak for themselves: in 1997, seven positive identifications; in 2001, 52; in 2004, 522.

Srebrenica has become iconic of Bosnia’s carnage, yet it tends to detract from other atrocities over the three years of the war. Bosnia is a country without a reckoning, a call to account. And nowhere is this more brutal than in Krajina, with the second biggest concentration of mass graves, the first of which was found in 1999 at Kevljani. Near what had been the iron ore mine of Omarska, the grave contained 143 bodies of men murdered in the camp.

A second grave was found at Kevljani, this one with 456 victims of camp Omarska, and others around a mining facility at Ljubija. But only in 2013 did Bosnia’s single largest mass grave away from Srebrenica come to light, a few kilometres away down a dirt track from Omarska: the grave at Tomašica in which Fikret Bačić found his brother.

It’s one thing to exhume bodies from a grave of 11 people, as many of them are. But here were 434, possibly more

The site looks like a pond now, water settled in the sunken earth, from which reeds grow. A family from the nearby Serbian village walk up the track towards it, carrying fishing rods. But for more than a year, this was a scene unlike almost any other in Bosnia since the war.

“They arrived here, all of them,” recalls Dijana Sarzinski, mortuary manager at the Krajina Identification Project (KIP) facility on the edge of Sanski Most. “It’s one thing to exhume bodies from a grave of 11 people, as many of them are. But here were 434, possibly more.” The figure would later rise to nearly 600. “They had been preserved in clay for 20 years, tightly packed together, glued by decaying tissue.”

Some people are identified on the basis of just a few bones, and it is up to the families to decide whether they have enough to bury or whether to wait for more. “People find their missing, but not complete,” says Amor Mašovic, who set up the Muslim-Croat Federation Missing Persons Commission after the war. “They may bury a few fingers and a leg, and five years later there’s a knock at the door, it’s the left leg now, two years later a piece of skull. It’s part of that awful limbo.” At one point, Islamic spiritual authorities decreed it irreligious to bury less than 40% of a body, further exacerbating the trauma for those believers trying to deal with fragments.

“During the siege of Sarajevo,” recalls Mašovic, the Bosnian Serb general Ratko Mladić told his gunners to “‘drive them to the edge of madness’. Well, this was the same principle, but for the madness to remain after the war. The madness of relatives of the missing, which will remain until their own deaths. They appear as statistics, all 40,000 of them. But each number is a horror story that people are going through, every one of them like a novel you could read for the rest of your life.”

There are organisations that form a bridge between the mechanisms of search and the families of the missing. Mirsad Duratović, a survivor of Omarska, is president of the Association of Camp Detainees of Prijedor 1992. He is also, as he puts it, “a messenger of death” to families he knows well. He brings news mainly of men he knew from Omarska, where prisoners would be called each night from their crammed quarters for routine torture, rape, mutilation and death.

“I’ve had every kind of encounter,” says Mirsad. “I ring the bell, and for 20 seconds we embrace, because no words are necessary. They already know what news I bring. You can’t explain or describe the feelings, nor would I wish them on anyone else. In a strange way, it’s the best kind of feeling, and the worst, at once.”

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We talk at the site of the Tomašica mass grave after yet another day of commemoration at Omarska, women laying flowers at the now-locked doors of rooms where they were kept, hardened men cracking up at the reliving of cruel memory. “The question of the missing is what made me come back,” says Mirsad. He had been living in Germany until 1999. “I had myself lost 47 members of my extended family.”

Mirsad wears pressed shirts and occasionally a suit, which is rare around here. His hair is groomed, he drinks little. When Mirsad talks about “the feelings”, he speaks from experience. “The hardest of all,” he says, “was to knock on my own mother’s door, and tell her that her own husband, my father, and her other two sons, my brothers, had been found. On one hand, it was the news we had all been waiting for; on the other, it is the beginning of a different grief. My mother always said that when her husband and my brothers were found, things will be easier. The pain of the wait was killing her, it had been so long. Now we have buried them, and we can at last start to go through the bereavement. That day I had to be both a messenger and a son.”

For Mirsad, there is a wider purpose in this work: “To prove that there was genocide in an area where there has been no conviction for genocide.” The war crimes tribunal in the Hague has ruled in serial cases that genocide was committed at Srebrenica and adjacent Zepa, but not yet anywhere else in Bosnia. “To prove systematic killings and prove systematic hiding of bodies. Systematic and premeditated. It wasn’t enough to kill all these people and their families and children, burn their towns and villages, blow up their mosques and Catholic churches, burn their files and papers and history, take the men to concentration camps and either kill or deport them, so these people never existed – and then to systematically hide their dead. If that is not genocide, I don’t know what is.”

The bodies from Tomašica, like those from all around Krajina, come first to that facility on the edge of Sanski Most. Forensic anthropologist Dijana Sarzinski’s role here is testimony to Bosnia’s global leadership in this expertise: from Sarajevo, she studied at Bryn Mawr College in Pennsylvania and then at the University of Central Lancashire in England, before coming home to join ICMP as an intern.

The standard operating practice here is the world’s “gold standard,” says Sarzinski. Remains are meticulously washed, and a biological profile established. Scientists and technicians work in silence, clad in blue tunics and masks, washing body matter, cleaning bones with toothbrushes. The bones are subject to a physical decontamination, followed by removal of any exogenous DNA that may have attached itself. A small sample of bone – a “bone window” – is then extracted with precision blades, to be passed on to the laboratories. “We’re not allowed, as anthropologists, to determine a cause of death – that’s for the pathologists,” says Sarzinski. “But we can prepare the cases and point out possible causes.” The single bullet holes through the skulls of these men from Hozića Kamen are articulate enough.

Facebook Twitter Pinterest The remains of people massacred during the Bosnian war are laid out on tables at the Krajina Identification Project in Sanski Most. Photograph: Andrew Testa/Panos

The numbers arriving from Tomašica were so overwhelming that the bodies “had to be treated with salt, basically mummified, using a method thousands of years old, preferred by the ancient Egyptians”, she says. “They had been preserved in clay for so long, and very quickly decompose – through dessication of tissue, bugs and maggots.”

The bodies from Hrastova Glavica, a desolate mountain hamlet near Sanski Most, presented a different challenge. In August 1992, the Serbs brought 125 prisoners here by bus. They took them off the buses bound in groups of three, gave each man a cigarette, shot them and slotted them individually down a crevice in the rocks. (The grave was found because one man broke free and survived to tell the tale.) “Body parts had been squashed together for so long, they were all co-mingled, tissue stuck together, tissue and bone from one body all mixed up with another,” Sarzinski says.

Once the bones are ready, they have to be re-associated with others from the same skeleton; this is crucial to establish what grave-robbing has taken place. “It became very quickly clear that of the 434 bodies we received from Tomašica, 56 cases of body parts needed re-associating with cases which had been previously identified at Jakorina Kosa,” says Sarzinski. But others enter into the category of NN, no name.

I ask whether Sarzinski would like to come to this year’s commemoration at Omarska – put faces to the bones, as it were. “I can’t,” she replies. “I have to do this job for what it is. I can’t afford to cross that line.”

Hava Tatarević’s garden is ablaze with the palette of summer. She is the older sister of Zijad Bačić’s murdered mother, and she survived – “if you can call this survival,” she says – to tell the story of the night she lost her family, some weeks before the murder of Zijad’s. “It was one of the first days of the war. Men came to the house, and took them all away. They took my husband, Murharem. And six of my sons: Senad, Sead, Nihad, Zijad, Nidzad and Zilhad. They came to the house with guns, and balacavas, and just said: ‘You’re coming with us, if not we’ll kill you all, here and now.’ I started to cry, and they said: ‘Don’t worry old lady, they’ll be back.’ And marched them down the hill.

“I never saw my sons again, but a few days after that, the same men came back and ravaged everything. They stole what they wanted from the house, ate and drank what was here, and smashed the rest. They killed all our animals. I was taken to the camp at Trnopolje, then on the convoys to Travnik. From there, after a long time, my sister came and took me to Croatia, and then Germany.”

Mrs Tatarević pauses. The only sounds audible are bees across the flowerbeds and the hum of a tractor in mid-distance. “I started looking for them all right away,” she continues after a while. “First of all I looked around the refugee camps. I wrote letters. People would come to Germany from all over the diaspora, and I’d ask them: have you seen my children? Have you seen my husband? I started coming back in summer to rebuild the house, and plant things. And I asked everyone, even the Serbian neighbours: have you seen them? Do you know where they are?

“I don’t know how I survived the pain. I don’t know if I did survive the pain. I just wanted to know – how did they die? Are they still alive? Might they come back to the village while I am in Germany? If they were dead, all I wanted to do was to hold their bones in my hands, and find a place of green grass under which to bury them, and say my prayers and say: there they are, my dead sons.”

Then, after almost a quarter-century, she learned about the new mass grave found in 2013. “I think I knew, I had a sense, something told me this was it. We went to Bosnia on the Sunday, to the grave site, to Tomašica. There was a grave full of children and young people from our village. And there was one of my sons, Senad, still with his wedding ring on. And I knew, even before they took the others to the laboratory, a voice told me it was them, the other five. And yes, later, a woman came: ‘We have your husband, we have your sons,’ she said.”

Dusk falls across the heat haze. The sound of the evening muezzin drifts across the valley – once intended never to be heard here again. The brutal bedlam of those days and nights 24 years ago seems unimaginable, but Mrs Tatarević’s gracious presence, and falling tear, make it only too real. “It is hard enough to lose a child, I think. But to lose them all? What can I say? What can I do? I cannot jump out of my skin into another. I just have to do what I can with my own. And they are buried now. The wait is over.”

• Additional research by Elsa Vulliamy and Victoria-Amina Dautović. A longer version of this article was first published by Wellcome on Mosaic and is republished here under a Creative Commons licence.

• Follow the Long Read on Twitter at @gdnlongread, or sign up to the long read weekly email here.

• This article was corrected on 6 December 2016. The subheading of the previous version incorrectly stated that the International Commission on Missing Persons was a UN organisation. As the article states, it is an independent body founded in 1996 on the basis of an initiative by President Bill Clinton.