Robert McNeil is a dapper, soft-spoken 67-year-old who lives in the West End of Glasgow with his wife Kathy in a house increasingly full of his own paintings and his memories of a remarkable career..

It was Kathy who bought him an easel as a retirement present. A reminder of a life long gone, a path not taken. Once upon a time - that time being the 1960s - McNeil had gone to London with his portfolio and a dream of getting into art school. He'd always loved art. And art galleries. In Glasgow back then it was where, he says, you went to try to find a girlfriend.

He had a talent too. It was his art teacher, a Mr Mackintosh - not related, McNeil thinks - who encouraged him to go to art school. Instead, he went to London and joined the counterculture. Ended up going on anti-Vietnam marches. It was a time when going to art school didn't seem so important. Eventually he dropped out and it would take him more than four decades to find his way back to painting.

When he retired he imagined he'd spend this time visiting galleries. The easel made him "dabble" with a brush and oils. "I really thought it was going to be just for my own enjoyment. It never in a million years occurred to me that anyone would be interested in them."

But people were. People are. Because if there isn't 40 years of artistic experience in his paintings there is 40 years of life experience. And some of that experience was hard and grim and painful. Because Robert McNeil has seen life at its worst. He's seen mass graves in Bosnia and Sierra Leone, he's helped try to identify the bodies found therein in a bid to bring war crime charges against the perpetrators. He's seen the worst that man is capable of.

In conversation McNeil is considered, patient and deliberate, the qualities you would want in a man in the field of forensics. And even though he is still, as he says himself, learning to paint you can see the same qualities in his work.

And yet the pattern of his professional life started off so accidentally. When he came back to Glasgow from London he took the first job he could get, working as a mortuary technician at the Western Infirmary. Unbeknownst to him he'd found his future. "I really became fascinated with the pathologists and trying to establish causes of death. And there were forensic cases that I found especially interesting and so I just literally lost any thoughts of a career in drawing and painting."

For the next 40 years he worked in the NHS and then in the wake of the Srebrenica massacre in July 1995 he was asked to go to Bosnia, to Tuzla - the nearest safe area to Srebrenica - as a member of the first International Forensic team. He'd seen a lot in his job. He'd seen dead men and women, dead children even. But what he was to see in Bosnia was of a different order.

The massacre saw the killing of more than 8000 Bosniaks, mostly men and boys, by units of the Army of Republika Srpska under the command of General Ratko Mladic, a name that would become all too familiar in the years to come. It was the worst crime on European soil since the Second World War, according to the UN.

"The brief there was to establish the causes of death because the UN was keen to capture the war criminals they had indicted and they needed evidence to ensure that the crimes committed were actually crimes against humanity or genocide."

This was most definitely the case. "Unfortunately though, the identification of those bodies was difficult because, unknown to the UN, the Serbs, realising that the bodies were going to be exhumed had gone into the graves, moved all the bodies. They'd torn them all apart with mechanical diggers and so on and then reburied them. So it turned out we were dealing with thousands of body parts rather than whole bodies and the remains would be in mass graves in different parts of Bosnia."

The team were based in a bombed-out clothing factory with part of the roof missing and with no electricity or water and within view of Serb troops on the hillside. "We were warned not to go to the back of the building when we were having our breaks because we could be seen by snipers."

Now and again the mortuary itself was threatened and the team was told to evacuate. But the work was ever present, a persistent stain. "We lived with families, with two women who had lost every male member of their family in Srebrenica. They'd been ethnically cleansed. So coming in from work that first time there was very little electricity or water. You couldn't help stink of death and although we were told not to talk about our work it was impossible for them not to know what we did. And so they were asking questions: 'Have you found my dad?'"

The work was grim. Containers with 200 often badly decomposed bodies would be brought to the forensics team in bags. They would be photographed and X-rayed before the post-mortem. DNA was extracted from the bones for identification and then examined for any sign of trauma. "There would be bullet holes and bullets still in bodies, so those were all removed for ballistics to assess."

There was no electricity so at one point McNeil had to use a hacksaw to open a skull, much to the horror of one onlooking police officer. "It's crazy and doesn't bear thinking about but for me it was a problem that needed to be solved," says McNeil.

The imagery of these trips would eventually surface in McNeil's paintings. A yellow bucket full of skulls. Protective paper suits and rubber boots hanging like empty ghosts. "I've been very careful not to show horrors," McNeil says. "I don't know if that is something I should do. I haven't made my mind up yet."

In the end, he says, there was a compulsion to reflect his past in his art. "I was experiencing I suppose what you'd could call a mild form of post-traumatic stress syndrome. Although it didn't bother me, I did have pretty vivid dreams. The most graphic one is me being trapped in a grave myself.

"They stopped and it was fine but there were images in my mind that continued with me in my waking hours. I started looking at a diary that I'd kept for various places I'd travelled to. I thought about writing about those experiences but instead just decided to paint one."

In all McNeil returned to Bosnia six times and had two trips to Kosovo, taking holiday time from his job as mortuary operations manager for NHS Greater Glasgow. He also worked on a First World War grave at Fromelles, a very different experience because the bodies were soldiers who had died in battle, buried respectfully by their enemies.

Over the last 20 years the experience gained in Bosnia has informed subsequent tragedies. McNeil was contacted by the Home Office on the day of the London bombings on 7/7 and was able to get 32 people to the city within 24 hours. In south-east Asia after the tsunami in 2004 one of his colleagues was in Thailand within 12 hours.

In short, systems are in place. And they work. In Bosnia the task of identification goes on to this day. Some, alas, will never be identified because there are no living relatives and so no post-mortem DNA match is possible. But thanks to DNA evidence around 6500 positive identifications of the 8000 Srebrinica victims have been identified so far. "The biggest forensic puzzle in history has been solved, which is quite momentous," McNeil says.

That is no longer McNeil's job of course. But maybe he has discovered another one. A year ago, after the first exhibition of his work at the Mitchell Library, he thought he would stop painting about his experiences and concentrate on portraiture. But he has since changed his mind. Next year he hopes his paintings will go on display at Holyrood. He is also in touch with the charity Remembering Srebrenica.

He's even recently returned to Bosnia, "a very emotional experience", he admits. "I met Husan Husanovic who escaped and survived the massacre and Mejra Dzogaz who after 20 years is still waiting for her husband and three son's bodies to be found."

The pain and the horror of those days has not gone away for some then. So Robert McNeil still has a job to do. He has to bear witness. In paint and in person.