Grace Lyttle is 14, with a Loreto Convent uniform, a ponytail of thick, fair hair and huge, troubled eyes. By the laws of nature and rampaging hormones, she should be driving her parents crazy, rowing about everything from high heels and homework to the need for industrial quantities of glitter nail varnish.

But it's been four months since Grace's last carefree, teenage moment, singing along to the Spice Girls' Viva Forever blaring from an Omagh record shop. They had roamed the town on that beautiful summer day, buying a glitter present, eating pizza. Yeah, there was a bomb scare. But come on. Who'd plant a bomb in wee, sleepy Omagh of all places? And why, when there was peace?

Then Viva Forever stopped abruptly.

"I got frightened then. I wanted to go home. I was looking straight across at the car and there was a God-almighty bang. It was like it didn't happen for a couple of seconds. No movement, no sound, just total silence. Then a scream . . ."

A few hundred yards away in his garden, Conor McCrory heard it too: "I'll never forget the noise. I thought it was seagulls screaming."

A body was blasted back towards Grace. She couldn't move until her friends forced her to run. "On my own, I wouldn't have . . . I felt there was nothing to go on for."

She stopped to pull glass out of her knee and again when her asthma forced her to slump into a bench. As she huddled there, fighting for breath, a young foreigner collapsed across the seat . . . Remembering, her voice becomes a crescendo of loss, guilt, pity, and terror: "He was only a young fella and he had a look of disbelief and pain on his face and he was holding himself around his body and he was looking up at me and I felt so small because here was me with my asthma attack and people were dying and screaming beside me and I couldn't help them."

To her left, a woman was placing her jacket across a body. A man was trying to get up, but his movements were "like a rag doll and his arm was the colour of something rotten". Another was babbling into a mobile phone: "I can't find them, I can't find them, I don't know where they are . . ."

And the low frantic murmuring she heard was grown people praying: "O please Mary, don't let this be real, please Mary, don't let this be real, please . . ."

Though Grace didn't know it then, in that tiny stretch of space and time, the lives of 29 people along with healthy, twin seven-month foetuses had been carelessly snuffed out by the so-called "Real" IRA.

Their victims were babies in the womb, toddlers, teenagers, mothers and fathers; Spanish, English and Irish from North and South; Protestant, Catholic and Mormon.

Brian McRory had been leaning on the car when it exploded; a few fingers were all that remained of him. All they left to Laurence Rush of his wife Libby, was a part of her face and a shoulder blade. A mother wanted to keep her son's coffin open but closed it because her boy looked "so unnatural", covered in glass and shrapnel.

Michael Gallagher, the gentlest and most ecumenical of souls, lost his beloved only son, Aidan: "Our one consolation was that Aidan's body was complete".

Fate decreed that Aidan, who had last walked through Omagh town on the previous Christmas Eve and had always press-ganged his sisters into buying his clothes, should go shopping for boots and jeans on Saturday, August 15th.

And thus, at 5.30 the following morning, his father became one of the grievously afflicted on Omagh's Via Dolorosa, the journey from the leisure centre to the makeshift mortuary in the army barracks, where the blinding lights, the hum of the refrigeration units and the smell of the dead is branded into their psyches.

It wasn't Michael Gallagher's first intimate brush with death. On the sideboard sits a photograph of his raven-haired brother Hugh, a young Roman Catholic in a mixed marriage who joined the UDR. Though he left the regiment not long after, the IRA stuck to its principles; it murdered him anyway in 1984.

They waked Aidan at home - the London Evening Standard asked to cover it to see what a wake was like - and a congregation of thousands turned out for the funeral, along with a bishop and seven priests. Presbyterian and Church of Ireland ministers spoke and were received with emotional bursts of applause. "And I felt, sitting there, this is right. This is the way that it should be," says Michael. "There is one God, that we are all praying to . . ."

Praying was what Omagh did that week. Day after day, thousands of sombre men, women and sobbing teenagers put on their Sunday clothes to walk the silent streets and criss-cross the narrow little country roads towards another traumatised household, wake or funeral - sometimes four in a day.

Running in parallel were the hammer-blows from the hospitals as the extent of the injuries was revealed; a 15-year-old girl had lost her sight; multiple amputations among children and adults; disfigurements beyond imagination.

Some 370 people were hospitalised with serious injuries, 60 of whom had fractures and multiples injuries. Twenty of these suffered amputations or will require a mobility aid of some kind. Many will be disfigured.

Grace Lyttle points wonderingly to a tiny red dent above her knee: "That's the only mark I have." It is Grace's mind that is crippled, tortured by flashbacks and by "something inside, like a big weight, and you can't lift it no matter how hard you try." She loved to sing, go out with friends. "I'd just as rather stay at home now."

At only 14, Grace is as much a prisoner of the Omagh bomb as the most cruelly injured. The trouble is that no one can bind her wounds or measure her suffering. "I'm alive but I've got all these scars in my head," she says, bemused.

"Patterns show that those most mildly physically injured tend to develop the most severe psychological injury," says Michael Duffy, team co-ordinator at the Sperrin and Lakeland Trust's new trauma centre. The reason is obvious: the mildly injured are left at the scene for longer, fully conscious, to witness the carnage and listen to the screams.

The truth of this is being borne out all over Omagh.

Apart from the 370 people hospitalised, local GPs had more than 2,500 contacts with mildly-injured victims in the first few weeks alone. Many of these are left with hearing problems, some with shrapnel pieces still working through their bodies.

They also include the children who become hysterical when the vacuum cleaner is switched on, who cling to parents and wake up screaming in the night; the adults suffering from flashbacks, sleeplessness, depression, bursts of anger and uncontrollable tears.

Weeks after the bomb, a man had his car valeted three times in as many hours to eradicate an odour detectable to no one else; the stench of burning flesh had invaded his psyche.

A mother, close enough with her two small children to be showered with glass, says they're "fine" but seconds later, is sobbing helplessly that more than three months on, her six-year-old daughter is too frightened to go back into town.

It took three months for Grace to accept counselling at the trauma centre and she only did it for her mother. There she found a rock in her counsellor, the "wonderful Grainne". Still, the bang of a door sends her hands flying to her head. "I don't think people really understand," she muses in a desolate little voice. "In the end, you're alone . . . and so lonely."

Elizabeth Capewell, an English specialist in crisis management and education and a veteran of the Hungerford and Dunblane lone gunman massacres in Britain, is working with the teachers of a 70-pupil rural school where three small children have suffered multiple bereavements in the bomb. The ripples of grief in this one tiny school extend to a host of their cousins and friends, to the teachers who were deprived of a friend and key helper, to the numerous cases of trauma and near-misses.

Mental trauma is all-encompassing, she explains. A scan of a traumatised brain even looks different.