Next Sunday, 18 April, will be Baylee Almon's 16th birthday. There will be a party and cakes, and her younger half-sister Bella and half-brother Brooks will be there. A place will be set for Baylee, but she will attend in spirit only, not in body. The day after her first birthday, on 19 April 1995, six years before 11 September 2001, Baylee was one of 168 people – including 19 children and babies – who died in a bomb attack on American soil.

The Alfred P Murrah building in Oklahoma City was bombed at 9.01 in the morning, as a normal working day on the Great Plains was getting under way – not by Islamic fundamentalists plotting in Afghan caves, but by a paramilitary unit of Americans who called themselves "patriots", led by a former serviceman and 1991 Gulf war veteran, Timothy McVeigh. McVeigh was executed in 2001, and his principal accomplice, Terry Nichols, is serving life.

"McVeigh was one of us," says Baylee's mother, Aren Almon-Kok, now remarried to a military veteran of Dutch descent who was wounded in that same Gulf war. "McVeigh was an American, like me and Baylee," she says, "and he walked into the building and saw the daycare centre where the children played – he knew they were there. Why did he do it? What was the point? There is no answer, and I've always said that the reason I wanted to see him die was nothing to do with closure, because there ain't no closure. It was so I'd never have to hear him explain himself, and justify what he'd done."

The fact that the 168 deaths at Oklahoma were the result of Americans killing Americans in the name of America has made the incident in some ways harder for the nation to process than 9/11 and the less complicated enemy, al-Qaida. "It made a terrible difference that this was homegrown terrorism," says Almon-Kok. "It left you with nothing to trust or believe in, apart from my faith that this city did everything it could in the aftermath, and that we have a legal system which, for the most part, works. But that doesn't answer why fellow Americans wanted to come killing our kids."

Perhaps this is why the Oklahoma bomb is not as centre stage in America's collective memory as it should be. When Al Gore was interviewed about the extreme right by Larry King recently, there was no mention of Oklahoma. Coverage of last month's arrests of militants belonging to an offshoot of the same Michigan militia that McVeigh belonged to omitted to mention the bomb, days away from its 15th anniversary. There is extreme awkwardness around this enemy within, but also current concern about reverberations of McVeigh's cause: war against the American government.

"What scares me," says Kari Watkins, executive director of the Oklahoma National Memorial and museum, "is that society now is configured much as it was in 1995. The militias are back, there's a language of extremism out there, of hatred against the government, threats of violence." Monday 19 April will mark the 15th anniversary of the bomb, but it is also the date chosen by the Tea Party, a right-wing activist organisation, for national protest, in all state capitals, against the current administration. In Oklahoma the Tea Party has bowed to the solemnity of the date, so that two events will occur on different days: one on 15 April, howling abuse at the government, and another on 19 April solemnly honouring the lost and the survivors. Nevertheless, there is local outrage. "I don't know how they dare," says Keith Simonds, a police officer at the core of the rescue operation in 1995, "spewing their hatred on a day of remembrance." No one is saying that the Tea Parties or their protest are comparable to McVeigh, "but all it takes is a few dangerous extremists out there to hear things the wrong way," says Watkins.

The Great Plains of America's heartland are no strangers to disaster: tornados, drought, prairie fires and of course the red plains turning to dust bowl in 1938, causing the "Okie" exodus west, registered into epic history by John Steinbeck's The Grapes of Wrath and the songs of Oklahoma's greatest son, Woody Guthrie.

But by 1995, Oklahoma City had become a reasonably but not ostentatiously prosperous state capital with an economy based on natural energy, in which the largest employers were the state and federal governments. So there was nothing in the universe to prepare Almon-Kok, the day after Baylee's first birthday party, for what followed. Nor Kathleen Treanor, who lost her four-year-old daughter, Ashley Eckles, and both her parents, Luther and LeRue. Reporting on the bombing for this newspaper, I met Treanor wandering beside the rubble on the night of the explosion, holding a picture of little Ashley, in vain hope. There was nothing to alarm Claude Medearis, a customs officer, as he popped by the office to make a phone call – which he would never make – on his way to an appointment elsewhere. Nothing to indicate that Florence Rogers' meeting with her staff at the Federal Employees Credit Union savings co-operative would be disturbed and 18 of her employees killed. The sky was deep blue above the prairie, until contaminated by a plume of black smoke at 9.01am.

Something had been festering in the American undergrowth. Calling themselves a militia, gatherings of extremely right-wing men (and some women) were crawling around the backwoods in camouflage fatigues playing war games against a loosely termed "federal government". Absurd, certainly, but the movement had a seriously dangerous core. Back then I explored this group and some of its more (preposterous) propositions: there should be an uprising against "The New World Order", of which the US government was a puppet; there was a site in Nevada where the militias believed a crashed UFO had been taken, the government plotting with aliens. The movement's leadership in Indiana were apparently preparing for armed insurrection, and a training compound in Arizona was headed by a man Timothy McVeigh knew, William Cooper, who threatened me when I tried to visit. In Cooper's 1991 book, Behold a Pale Horse, he insisted that the prison transfer centre in Oklahoma City was a "concentration camp" for those resisting the New World Order of the Antichrist. In November 1994, on his radio station, Cooper issued a call to arms: the militias should be ready, he said, to "fight a war" within six months. Closer to hand, at another compound called Elouhim City – visited by McVeigh – the leader of the Oklahoma militia, Robert Millar, urged his followers to "take whatever action necessary against the US government". A few weeks later, McVeigh and Nichols parked a rented Ryder truck packed with 2.15 tonnes of ammonium nitrate and fuel oil outside the Murrah Building, lit a fuse and walked away.

Baylee Almon had a smile like a spring daisy, and before she and the 167 others were killed by the ensuing explosion, this city of the plain was wrapped in glorious pear blossom, as it is this Eastertide, time of Resurrection in this deeply religious city, and of birth on the land beyond. In 1995, 19 April followed Easter Sunday by three days. But that smiling infant was not the child the world came to know – it was the baby cradled in a fireman's arms, lifeless, in what emerged as the enduring image of the attack. "That photo took on its own life," says Almon-Kok in her official video-recording at the memorial, "which was horrible for me, because after a while I couldn't ever remember Baylee being alive." Now, says Almon-Kok, "we tried to get a handle on it in the courts, get it off of T-shirts and things, but we were turned down because the judge said Baylee was not a public figure before the attack" – thereby not protected by privacy laws. "If she'd been Michael Jordan, we'd have been OK," says her husband, Stan Kok. "That is not my child, in the picture," insists Almon-Kok. "My child was a beautiful little girl who loved to play."

But the picture was not the only obstacle on Almon-Kok's road of reckoning. "People kept saying: 'She's in paradise, a better place', and I said 'NO, she should be here with me'. Others asked: 'Why were you a single mother? Who's the father?'" There was even "a weirdo in California who became obsessed with Baylee, sent letters to her grave and moved to Oklahoma to be 'near her'. He knew where we lived". The family took a court injunction to ban the grave-stalker from the state of Oklahoma. Aren Almon was lucky to meet a fine man in Stan Kok, who says of these supplementary tribulations: "I've got to the point when nothing surprises me any more, and that's a sad world, when nothing surprises you."

But of all the endurances, says Almon-Kok, none struck harder than "people asking me why I left my child in a nursery in a federal building. I said I expected my daughter to be safe in a federal building." And it was this which took Mrs Almon-Kok on a remarkable crusade: to ensure the installation of reinforced, shatterproof glass in all federal nurseries and eventually all federal buildings.

In September 2000, President Bill Clinton signed into law congressional bill number HR 4159, better known as Baylee's Law, which ensured glass protection that might have saved an incalculable number of lives on 19 April. "I suppose I believe that bad things happen for a reason," says Almon-Kok, "and that my loss could save other lives." So that "when 9/11 happened, I watched those people carrying their pictures of their loved ones, looking, and thought: 'I've so been there.'" But it was only later, visiting New York and Washington, that staff from the Pentagon came to thank Almon-Kok for their lives: "Their section of the Pentagon, near where the plane came in, was the only section where the new glass had been installed."

But Almon-Kok's grief is now more private, at home near Choctaw, on open prairie. "Things had to change for me and the children. Bella was used to being with me on the road campaigning – eating at 2am meant nothing to her. She grew up climbing all over furniture in the White House. But Brooks wanted to be in bed at 8 o'clock, and that calmed things down for all of us. Some people kept what happened from their children, but I wanted them to know. They feel her presence: Brooks had a project at school called I Have a Dream, and he wrote: 'My wish is that Baylee didn't have to die.'"

By 1995, 23-year-old Catherine Alaniz had tried her level best to recover from her husband's death in the 1991 Gulf War. When Andy Alaniz was killed by "friendly fire" on the day of the ceasefire in Iraq (which the Pentagon initially denied), Catherine, aged only 19, had been six months pregnant with her daughter, whom she called Andee. Inevitably, her mother Sharon and father Claude, a US customs agent, had been bedrocks of strength for the young widow.

On the morning of 19 April Catherine took a call: "Did you feel that?" There had been an "explosion downtown": the caller mentioned the federal courthouse. "My dad worked downtown," Catherine recalls thinking, with a shudder. "I called his office number, but it was switched through to Houston, like happens at night, even though it was after 9am." Catherine rushed to her parents' house, to find "my mom sitting on the floor, rocking back and forth with the phone in her hand, waiting for my dad to call. I sat down next to her, wrapped my arms around her and I told her, 'Dad is going to be OK. God wouldn't do this to both of us.'"

It took several days for both women to learn that another husband and father, aged only 41, had now been lost. "Evidence had to be collected," says Catherine. "Then a woman in hospital who'd survived, her name was Priscilla, told us dad had gone by the office to make a call. That he'd been there at the time of the bomb. So we knew, but not officially. Next day, we went to the funeral of the other customs officer killed, Paul Ice, and the pastor said, 'Hope for the best', but we already knew, and next day it was confirmed."

"I've thought this through," says Catherine, "and I've decided that the only reason God would take my husband would be so I could help when Mom lost hers. Mom wanted to get involved immediately, in the committees, and the trial, to separate herself, emotionally."

The trial of Timothy McVeigh in Denver, and the long road to his execution was a Calvary for the victims and survivors. At one point, the judge ruled that victims could not observe the proceedings if they wanted to make impact statements in the event of a guilty verdict. As McVeigh neared his execution, media developed a grotesque fascination with him. Gore Vidal corresponded with the mass murderer, and a book came to be ghost-written in which McVeigh dubbed the dead children "collateral damage" and those he had bereaved "the woe-is-me crowd".

For Catherine Alaniz, the trial was at times too much: "McVeigh sitting there, smirking at us, and we not being allowed to cry, show emotion or wear buttons and ribbons", in guidance from the judge. "Sometimes I had to leave the courtroom. But I stayed on in Denver with Mom and got involved in a local Catholic church, making hundreds of ham and cheese sandwiches for the homeless. And they had a programme visiting people dying of Aids, and I met a man, a Hispanic man, who was dying and I hugged him and he said he hadn't been hugged for years – it turned out he was from Oklahoma City." Of course, there was also downtime for the bereaved families to talk: with each other, with survivors and with those who had rescued them.

Police officer Keith Simonds was among them, anxious to give his impact statement after the verdict. Officer Simonds of the Oklahoma Swat team had clocked on for duty at 7am on 19 April 1995 and was answering a call north of the city when he heard the explosion, saw the plume of smoke and rows of his colleagues' cars parked near the federal building. He took a radio call "from the east side of the building: 'We've got wounded here,' it said. 'I'm in the basement. You need to shut off the water, it's flooding down here.'" After failing to persuade a fireman to accompany him or give up a flashlight, Simonds descended into the water: "It was a wreckage, the whole thing had come down, and we were crawling over debris to our victim, a lady called Sharon Littlejohn, trapped. Tyres were exploding, the water rising, and I said to my colleague, you take the legs, I'll take the upper torso. The water had been ankle-deep, now it was knee-deep. As we reached the surface I told her, 'Can you see the light, ma'am? It's going to be OK', and her arm suddenly hit me – I'd been holding onto exposed bone."

Officer Simonds' rescue of Ms Littlejohn became another iconic photo-image of that day. "[But] I had to keep working, I had to keep going," says Simonds, unaware then that the picture had been taken. "I was walking through the children's playground, that'd become a morgue – it was real hard that where the children played was where they were now laid out dead, in that biddy playground." During his incursions into the building, he said, "I saw an officer carrying a girl with a white dress with roses on it. I thought 'that's a good omen', and that she was alive. Later, I realised it was a white dress with blood on it, and she was dead."

Officer Simonds was told by prosecutors in the McVeigh trial that he should not give his impact testimony: "The judge and jury had heard so much, it might have backfired, they told me. As a police officer, I understood. As a man, I was pissed. So I really wanted to give my statement in the Nichols trial."

Ready to do so, and on the plane back to Denver for the Nichols trial, a young lady two rows ahead of Simonds was trying to get volunteers for a Christmas party for the homeless being organised by a church near the courthouse. "I thought she'd never stop," recalls Simonds. "What about some of you officers helping out?" suggested the young woman. "Next day," recalls the police officer, we went to a teahouse, and the same lady told me, 'We'll make sure you get to do your testimony', and I did." There was something about her, thought Officer Simonds, and the feeling was mutual. Her name was Catherine Alaniz, now Simonds: the rescue cop had met the woman he would before long make his wife.

Home for the Simondses and their merged families is now a house in the Oklahoma suburb of Noble – a household of chattering teenage girls and visiting older brothers, grace before dinner, gazebo in the garden running down to a creek and a cabinet in the sitting room in tribute to the lost men in Catherine's life, and the officer's bravery that day. "We live in this world of quick fixes," says Catherine, "but there's no such thing. You can't plan when the loss will get you. There's no reason why the 15th anniversary should be any easier or harder than the 5th, or the 14th."

Keith Simonds, when he is not on the beat, protecting dignitaries or riding his Harley-Davidson, gives classes to children on the impact – and avoidance – of violence. "I tell them my story and tell them to report violence: I tell them if the third man in the plot, name of Fourrier, had told the police what McVeigh and Nichols were up to, I wouldn't be here today. I tell 'em to watch for gangs, to do sports or clubs or cheerleading instead; I tell even the baddest of them: it's OK to have feelings." "The thing I cannot explain to myself," says Catherine, "is that it was an American who did this, who had walked by the children he killed. You lose your false hope that it cannot happen here. This wasn't someone from the Middle East – an American did this to us, and it can happen again."

There was something unique about Oklahoma City's response to the bombing, which one sensed immediately at the time. The Okies love where they live with good reason: theirs is probably the least pretentious city in the world. They have down-to-earth decency, a rugged decorum, easy-going diligence. These characteristics came into play in the hour of horror, loss and need. Ted Wilson, chaplain to the fire department, recalls the rota of fast food chains and local pizza joints providing endless free meals for rescue workers and the vigils of those who hoped for news of loved ones, often in vain. He remembers round tables of local business donating cash for all reasons. "If word went out that they needed batteries at the site," he says, "a truckload of batteries would arrive." Rescue workers from out of town would return to their camp beds to find their clothes anonymously washed and ironed. People literally gave the shoes off their feet; local TV became an emergency needs swap-shop, often having to appeal to citizens to hold back as blood donation banks became saturated.

The aftermaths of outrages such as 9/11, Beslan or Srebrenica are characterised by grating and grated nerves – friction, even – between the differently traumatised bereaved, survivors and rescue workers. The different wounds cut so deep, and so unforgivingly to the individuals affected, that often these constituencies, though bonded by the same horror, are as divided as they are united by raw emotion and grievous loss. "It wasn't easy at first," says Catherine Simonds, "People would say to me, 'Well, I lost a child', and I'd think, 'Well, my dad was my grandma's baby.'" Almost uniquely, Oklahoma has suceeded in bringing these people together into what Rashell Hammons, the mother of children who survived – faced by those whose children did not – calls "an extended family". This is a far more formidable and singular achievement than it will appear when the bereaved, survivors and rescue workers gather together tomorrow week, and it did not happen by accident.

The anniversary commemorations are held at the Oklahoma National Memorial, a place most of the bereaved and survivors say they love, and in which they feel at home, helped, comforted if not healed. "I love it," says Catherine Simonds, "it is very, very beautiful." In a time such as ours of much thought and debate over what memorials and monuments should represent and achieve, this site sets a gold standard. It is unrivalled anywhere. The Oklahoma memorial is in part set on the gaping space left by the Murrah building, but includes some of its jagged, incinerated walls. There are 168 empty bronze chairs set on a lawn, each engraved with a name, their bases illuminated during darkness, so that the chairs of the dead float on beacons of light. At either end are huge "Gates of Time" marking 9.01, the last moment of the city's innocence, and 9.03, "the moment we were changed forever", says the guidebook. This place is always open, and the bereaved come often, as do survivors – there is always someone to tell their story to a visitor. Passing teenagers instinctively switch off their boom boxes; the 24-hour toilets are always clean. "It's not a cemetery, where people come to mourn, it is a place to remember and to learn," says director Kari Watkins. "A memorial should be beautiful, it should be a place that's always open, 24 hours, at its best for anyone who lost a family member, or even just those passing on I-40 who come by to look." The space is quite a lesson to post-9/11 New York, whose memorial committees have serially visited.

Inside, the Oklahoma museum is a journey from the moment of normality at 9.01am, through the explosion, chaos, claustrophobia of entrapment, the rescue operation, arrests and aftermath – all narrated by the videotaped testimony of living bereaved and survivors, and ending with redemptive light and a window overlooking the chairs of the lost.

The driving forces behind the memorial, Watkins and her visitors' services director Joanne Riley (who spends as much of her time coordinating the survivors and bereaved), decided that their project would in itself become part of the process that brought those people together. As a result, opposite the chairs across a reflecting pool, the "Survivors' Tree", an American elm that miraculously survived the bombing, remains, surrounded by the "Rescuers Orchard", smaller trees literally rushing towards it.

Involved from the beginning was Florence Rogers, chief executive officer at the Federal Employees Credit Union, a co-operative banking system. That morning, in preparation for an audit of her bank, she called a meeting for 8.30 in her office on the third floor. "I remember to this very day a girl called Claudette Meek saying: 'Look at all the primary colours we're wearing today, we look like a basket of Easter eggs.' So I remembered what they were wearing, and had no idea how important that would be when it came to indentifying the bodies." One of which was that of Claudette Meek. At 9.01, says Rogers, "I had just read an item off the list and turned – and bang – I could see clearly through the far wall. The floor had gone clean away, down into what we called the pit, leaving me alone on 18in of floor that had held. I know it sounds stupid, but I thought 'Oh my God, this is like a bad movie, and when it's all over, I get out of here.'"

Rogers was rescued, through a window, but that was the beginning of her grief. "They would bring me the purses, a girl called Robin Huff's – it contained ultrasound photographs of the baby she was going to have. Her little husband was so sad to see them; he was a man who had grown up with my boys, since cub scouts. I had 33 employees and I lost 18 of them. They were my girls, my family. Every Christmas, they would come here for a party – champagne and presents. That's when I miss them all, but I can't grieve for 18 people at once – each of them has their day. Something will happen, like one day I found a Santa's sleigh up there on the shelf, and that was Claudette's day. One day I saw a girl just like Sonia, eating a donut like she did, and that was Sonia's day."

Also up on the third floor that morning, "where my bank was", was Ted Wilson, pastor at Grand Boulevard Baptist church and part-time chaplain to the Oklahoma Fire Department, now ministering full-time to fire officers who, he says, "are still coming, after 15 years, suddenly traumatised, their brains finally processing this information, after all this time". Wilson is also a qualified intermediate paramedic, and objected to "the bodies of the children being laid out in their playground – a bad omen for later. They said they had to be close by the building, but we did move 'em to a parking lot." Before long, Wilson had "gotten in through a skylight to be working to release a trapped woman, still alive, on the third floor where my bank was. I was working with a tall state trooper, and the scare came for a second bomb and for us to evacuate the building. The trooper said he wasn't leaving the lady, and I said, 'Well, if you're not going, I ain't either.' So we kind of had the place to ourselves – and I located the lady's purse on top of the desk. Now there's two things a lady needs to be accounting for, her hair and her purse. Well, her hair was pretty messed up, but I said: "Ma'am, you needn't worry 'bout your purse, and you needn't worry about yourself either.' We became friends after she was released from hospital, Nancy Ingrams by name – she passed away just last year."

Wilson's job quickly changed to that of counselor and chaplain, and has not changed since. "I'm treating guys now, 15 years on, who've been treading water all that time and it suddenly hits. They were working for eight hours through rubble to reach a victim who has died. And if you do that, you personalise them on the way. You find their crucifix, their purse, pictures of their children, personal items. You are spending time with that dead person. You're saying: 'I must rescue the kids, but please don't let me be the one to find them.' You're finding body parts, and what we are saying is you have to think of it as reconstitution of a person, you're scraping body matter, but you're not a ditch digger. We tell them they have done the best they can to restore as much of that person back to their family as was possible."

On Monday week, 19 April, the national Tea Party protests will be held across America, venting anger at the Federal government. In Oklahoma City, however, their protest will be held four days earlier, so as not to coincide with the memorial at what was the site of the federal government building – almost an admission by the Tea Party that they are playing with fire. "They'll say they're coming here to protest Obama's health care programme," says Keith Simonds, "but that's not what it is. They're here to spew their hatred, vomiting their political agenda. How dare they come here?"

"One purpose of this memorial," says Kari Watkins, "is to teach the impact of violence. On Easter Monday, our biggest effort this year becomes law, against considerable opposition: that the story of the bomb be entered into the school curriculum for the state of Oklahoma. We need to teach this story, especially right now, when in America there are a lot of the same movements that were around in 1995, there's a similar mood, a polarisation. Look, no one wants the government on their backs – who does? - but all this alarms me, and we must work against it by teaching the consequences of violence against the language of violence."

The day of Watkins' interview, nine people appeared in court in Detroit, members of an offshoot of McVeigh's Michigan militia called Hutaree, charged with "seditious conspiracy" to kill a police officer and then bomb the funeral cortege, in order to spark insurrection akin to that sought by McVeigh. The previous week, congresswoman Louise Slaughter, who voted for President Barack Obama's health care reform, received one of many threats of violence to elected representatives, this one pledging that snipers would "kill the children of the members who voted for health care reform". Such language makes the blood run cold in Oklahoma; and the fact that most people in Oklahoma are deeply conservative makes the irony of both the bomb and their disgust at this language all the more cogent.

A warm evening breeze strokes the garden of empty chairs, while a late sun illuminates their bases against the opacity of oncoming dusk. As twilight falls, so does a haunted serenity, and silence – but for evening birdsong and the hoot and rattle of a freight train passing through town and out across the plains. The memorial's lights come up in phases, so that first the great Gates of Time shine out, reflected in the shallow pool between them, then the chairs of each person killed appear to hover on lamplight.

In the second row of chairs, indicating the building's second floor, 15 chairs are smaller than the others, commemorating the children. At the end, slightly apart, is that inscribed "Miss Baylee Almon", whose mother Aren's lovely face nevertheless wears the scars of grief until dispelled by a smile, at a joke by Bella or Brooks, or a memory of Baylee alive. Recently, Bella choked on some beef jerky and Aren was worried she'd been alone, but Bella said: "It's OK, Mom, Baylee was here." On Monday week, Bella will, for the second time, read out the names of the children killed in 1995, in reverse alphabetical order, so that the sister she never met, but whose birthday she will have celebrated the previous day, will come last.

And there, standing at a distance from the others, is the chair in honour of customs agent Claude Arthur Medearis, for whose daughter the bomb was a very different kind of fire from that which killed her first husband – though also from an American military man. "My husband had driven a Bradley military vehicle in the Gulf war," says Catherine Simonds. "Timothy McVeigh also drove a Bradley vehicle in the Gulf war. My husband died, but McVeigh lived. Then he came to Oklahoma City four years later and killed my dad. What am I supposed to think about that?"