The police Swat team which finally brought out the bodies were tough guys - many had served in Vietnam. But 'they were weeping and crying at what they saw,' said District Attorney David Thomas.

'The school's in a panic, and I'm in the library,' a teacher called Peggy is heard saying on a tape released by police on Friday. She is talking to a police dispatcher, begging for help.

Peggy, who survived, says desperately: 'I've got students down.' Then she says: 'Under the table, kids!' as she turns her attention back to the pupils who, moments before, had been studying in the Columbine High School library. 'Kids, under the table. Kids, stay on the floor... Oh, God. Oh, God - kids, just stay down.'

Harris and Klebold killed a teacher and 12 fellow students, 10 of them in the library from which Peggy placed her call, before turning their guns on themselves.

Bae Gottini, a 15-year-old with braces on her teeth, said it was not 'random shooting' - it was worse than that. Bae was cowering under a table with her friend Cassie Bernall when Harris - with 'a dumb giggle' and covered in blood - took Cassie by the hair and pulled it, handgun to her head. In the yearbook photo, Cassie wears a smile and a cross; Harris knew she was religious and had just been to Britain with her church. So for this murder he elected to play the Almighty.

'Where's your God now?' he jibed. 'Please don't kill me,' pleaded Cassie. 'I must be your God, because I'm in total control,' replied Harris, pulling tighter on her hair. 'You are not my God,' said Cassie. 'Tell me I am your God,' said Harris. 'You are not my God.' And with that he blew her brains out.

Another girl, Rachel Scott, was also hiding behind the table but looked over the edge to see a bloodied, giggling face staring at her. 'Peek-a-boo!' he laughed, then shot her.

For the slaying of Isaiah Shoels, Harris lived another of his miserable fantasies. Not God this time, but Adolf Hitler, whom he admired and whose 110th birthday it would have been on 20 April.

Isaiah's yearbook picture shows him with a puckish grin; he wanted to go into the music business. Despite heart problems as a child, Isiah was a remarkable athlete - a prime qualification to become one of Harris's victims. And he was one of the few black kids at Columbine, which had led to trouble with Harris and Klebold's gang, the Trenchcoat Mafia.

'Isaiah,' says fellow pupil Wes Lammers, 'was shot because he was black'.

But not immediately. Another witness, Craig Scott recalls Harris calling to Klebold: 'Hey look, there's a nigger over here.' Bae Gottini remembers either Harris or Klebold stamping a heavy boot on Isaiah's throat, pinning him to the floor. 'They were jeering at him, like he didn't belong in the community and didn't deserve to live.' And, pointing to Isaiah's slightly jutting forehead: 'Look at this black kid's brain! Awesome, man!' After five minutes of this filth came the bullet.

Investigators believe the pair may have wandered through the school after their initial attack in the library for up to an hour before returning there to kill themselves.

They also strongly believe they had help in preparing for their attack. They brought into the school an arsenal including a TEC DC-9 semi-automatic assault pistol, two illegal sawn-off shotguns and a short-muzzle carbine military-style rifle (used by two students in another schoolyard massacre last year in Jonesboro, Arkansas.) They also brought more than 30 homemade explosive devices and may have posted their plans to kill on the Internet.

Thirty-six hours after the outrage, Isaiah's father Michael is sitting on a bench in gently falling snow, a burly, bearded, kindly man wearing glasses and a Dallas Cowboys football jacket. He had, he says, told school officials about his son being bullied before by the 'Mafia'; nothing happened.

'I'm hurting inside,' says Mr Shoels. 'My son had no enemies. Hey, if you hated my son, you had a problem... And I'm upset at their parents. If my child had six butane tanks in the garage, don't you think I'd want to know abut it?'

No, sighs Mr Shoels, he won't go to the counselling sessions for the bereaved. 'I'm a Christian man, and I'm going to get over this. Everything happens for a reason.'

But what reason? America is still grappling to find language to speak about Columbine High School, where so many nightmares collided.

There is the nightmare of racism. The slaughter came on Hitler's birthday. Last year, the city of Denver was the arena for a spate of attacks - two fatal - by skinheads on black students. In the early Nineties. this was the city that spawned 'The Order', a neo-Nazi gang whose leaders were imprisoned for the assassination of a Jewish disc jockey.

The nightmare of guns: the National Rifle Association is due to hold its conference in Denver this week and debate will rage at high volume.

There is the nightmare of a series of schoolyard shootings: in Oregon, Mississippi, Pennsylvania, Kentucky, Arkansas - and now Colorado.

The nightmare of satanism among the young: the Colorado killers shared the cheap thrills of the black occult with their predecessors who shot their classmates in Jonesboro, Arkansas.

There is the nightmare of cliques and gangs. America found a new monster this week: the 'Trenchcoat Mafia' with its black tunics, long black 'Duster' trenchcoats, berets, and sunglasses even in class. They greeted each other with the Nazi salute instead of a high five and kept to themselves.

There is the nightmare of violence in mass popular culture. The parents of the dead in the Kentucky schoolyard massacre made history this month by sueing the makers of the film The Basketball Diaries in which a trenchcoated Leonardo DiCaprio mows down schoolmates.

The Kentucky killer cited the movie as having inspired him, and Basketball Diaries was a favourite of Harris and Klebold too. There are violent video games, Doom and Quake, whose designers and producers are also being sued by the Kentucky parents.

Above all, for Harris it was Marilyn Manson: the infamous 'Antichrist Superstar'.

The cross-gendered, demonic deviant was due to play Denver next weekend but cancelled and sent his 'condolences to the students and their families'. Manson was Harris's abiding obsession; his favourite tracks were 'Irresponsible Hate Anthem' and 'Beautiful People'. He and Klebold had tickets for the gig.

'Hey victim, should I black your eyes again? / Hey victim, you put the stick in my hand / I am the -ism, my hate's a prism / Let's kill everyone and let their god sort it out / Fuck it - everyone is someone else's nigger.' ('Irresponsible Hate Anthem').

And yet none of these nightmares alone explains what happened. Eric Harris grew up in rural New York state, at Plattsburgh Air Force base not far from the childhood home of Oklahoma bomber Timothy McVeigh, alongside whom Harris will take a place in history.

His father was a decorated airforce major and the young Harris an exemplary all-American kid - a Little League baseball star.

The family moved to the prosperous Denver suburbs three years ago after Plattsburgh closed and Major Harris retired to a $200,000 home in a quiet tree-lined street in Littleton, wedged between the Rockies and America's second-fastest-growing city.

The adolescent Harris continued with his sports but played in the shadow of elder brother Kevin, the gregarious athlete, who became a 'Jock', one of the sporting elite.

Although a shy boy, the girls noticed Eric and he was friendly to those around him. He was 'well, cute,' recalls Kimberly Teel, now 18. He was also strange. One of his early dates, Tiffany Typher, came to his home one night to find him 'sprawled on the ground by a large rock, with blood splattered everywhere, as if he had bashed his head on the rock... It was fake blood'.

After a year, Harris befriended Klebold, who came from a more affluent background. Klebold's father is an eminent geologist living in a $400,000 house built into spectacular rock formations at Deer Creek Park in the Rocky foothills. Dylan drove his own black BMW. Young Dylan was, says a friend Nick Baumgard, 'a follower type, always looking for someone to lead. Harris fulfilled that role'.

Suddenly, both boys gave up sports to nurture their passion for computers. 'Eric was so smart that if the teacher had a problem with a computer, he would ask Eric to help,' says classmate Tom Kridle.

He and Klebold challenged each other in more and more violent computer games. 'He said less in philosophy class; he pulled away and got in with the outcast crowd', says Kridle.

The 'outcast crowd' was a self-conscious minority in a school full of clans and cliques - that was how you defined yourself. As Kridle explains: 'The 'Jocks' and 'White-Caps' play sports, the 'Preppies' are real straight, the 'Stoners' smoke marijuana and like Pearl Jam, the 'Hicks' dig country music'.

Athletic prowess was highly valued, and the 'Jocks', including the adored Kevin Harris, were therefore the elite clique.

To counter the taunts of the Jocks, the 'outcasts' formed the counter-cultural 'Anachronists', who were also computer fanatics.

Then a pupil called Joe Stair, who graduated last year, turned this clique into the Trenchcoat Mafia, which Harris joined 20 months ago. 'He clung to this group because they accepted him,' says next-door neighbour Karen Good.

The 'Trenchcoat Mafia', about eight in all, were labelled 'gay' and 'inbred'. There were rumours - still circulating - that close sexual ties did exist between the Mafia's members. Harris and Kiebold closed ranks with the rest, about eight in all, and began wearing only black. 'He started to get really fired up when you talked about Jocks,' recalls Tiffany Typher.

The Mafia cultivated its Nazi mystique, blended it with anarchism and nihilism, took industrial quantities of drugs and spent lunch hours together under a flight of steps, never in the canteen.

But, says Stair, he is appalled at what happened. 'They were very nice people,' he said of the killers on Thursday. Another member, Kristen Thiebault, says now that 'we're all completely sick. We honestly never thought anyone could do that, that we know.'

'We did stuff just for fun,' says Stair. 'Videos, role-playing, computer games.' One favourite game was 'Dragons and Dungeons', a drama involving apocalyptic fantasies from medieval times.

Harris and Klebold were not doing things 'just for fun'. Chris Reilly was in the same video production class, and remembers their effort: it was a dry run for Tuesday's carnage. It showed them, he recalls, stalking the corridors of the school with guns, shooting 'Jocks'.

By last year, Harris had shed all his friends beyond the 'Mafia'. Friends like Brooks Brown with whom he had hung out and played sports since the second year at school. 'One day, he suddenly threw a chunk of ice at the windscreeen of my car and broke it,' says Brown. 'That was the first I saw of his dark side.' Brown complained to Harris's parents, after which Harris threatened to kill him. Later, Harris posted a message on his website urging anyone interested to 'hunt down' and kill Brown.

Brown's worried parents went to the police, three times, to alert them to Harris and the mafia's website. They were ignored. 'The police were warned,' says Brown, 'they knew. They didn't do anything, and because of that, people have died.'

The Trenchcoat Mafia was meanwhile learning how to make pipe-bombs, and posting the recipe on the Internet. The websites became rabid - Harris's own site was spotted by the Simon Weisenthal Centre, which monitors Nazi propaganda. It boasted about 'shooting everyone' at Columbine High; it showed a montage of a gunner brandishing a Tech-9 automatic pistol, banned in 1994 but still the main weapon of choice last Tuesday.

Harris got work on a fireworks stall and asked to be paid in fireworks, which he would then detonate while at his other job in the Blackjack Pizza parlour. He and Klebold were arrested - for breaking into a van and stealing $4,000-worth of computer equipment. They were sent on an 'Anger Management' class.

Some chance: for propelling much of this, agree all of his peers, was Harris's immersion in the angriest and most deranged voice in American counter-culture, that of Marilyn Manson, the self-described satanist rock priest plus German 'industrial' bands such as KMFDM, which stands for 'Kein Mehrheit fur die Mitleit' - 'No Pity for the Majority'. Harris began to reply to the Jocks' taunts in German, and to plan a 'prank' on Hitler's birthday, 20 April. .

Recently Harris had seemed, says Brooks Brown, to have 'cooled down'. 'I told him "let's bury the hatchet" and he said "that's cool". He started being cool and making jokes and being nice.' Neighbours, however, report a flurry of nocturnal activity, including bottle-crushing to make shrapnel. Still, no one raised the alarm.

At Columbine, students get to telecast a 'message of the day', and on the morning of 20 April it read: 'Bet You Wish You Weren't Here. 4-20. 4-20. 4-20.' Brooks Brown went outside for a cigarette and saw Harris getting a bag out of the trunk of Klebold's BMW. Harris, he says, called over: 'Hey man, Brooks, I like you. Get out of here. Go home.' 'I went to have my cigarette,' recalls Brooks, 'and heard the gunshots... He saved my life, basically'.

While Brooks pulled on his cigarette, the evils that have recently ravaged poor-town schoolyards in the South now assailed cushy, conservative, Jefferson county, Colorado. 'I don't understand,' says DeForest Stalls, who runs a youth centre nearby, 'why people think money can prevent violence.'

By coincidence, the philosophy professor who wrote the seminal study on the satanist American underground - called Painted Black - teaches at Denver University.

Professor Carl Raschke has become America's leading secular critic of what he calls the 'psycho-therapeutic la-la land tolerance' of 'what is no longer an aberration, but a phenomenon of neo-Nazi terrorism with just as much of a religious basis as Islamic terrorism or Christian fundamentalists blowing up an abortion clinic.'

'What happened in Denver,' he says, 'was as planned and coherently motivated as an attack on a bus in Israel.'

Raschke sees the growth of the occult counter-culture as 'a mix of Nazi chic with Anton LaVey's "Church of Satan". It's a pop outgrowth of the decadent and occult movements of the last century in France, Germany and England, which formed the basis for Nazism. Same nihilism, same sordid fantasies.'

'The problem with American democracy,' Professor Raschke concludes, 'is that we just don't like setting limits to unacceptable behaviour - even when it's playing with political and spiritual nitro-glycerine.'