With the bodies of the dead still strewn about the library and cafeteria, bomb specialists worked to defuse some 30 bombs and booby traps hidden in knapsacks, sports lockers and vehicles so that investigators could take their first look at the horrors within.
After some confusion about the death toll, 15 people were finally said to have died and 28 injured in the four-hour spree of violence and satanic laughter. Sixteen people remained in hospitals in the Denver area after treatment for gunshot and shrapnel wounds.
Seeking to establish a motive for the deadly attack, police also seized bomb-assembly equipment, computers, weaponry literature, apocalyptic books and videos from the home of one of the two dead suspects, Eric Harris.
Harris, 18, and his fellow suspect, Dylan Klebold, 17, were said by their classmates to be introspective, defiant students who expressed their obsession with death and violence in school essays, poems and classroom discussions. Nobody, it seems, expected a tragedy of this nature, although plenty immediately suspected them once the shooting began.
"He [Harris] did it because he hated people," said Brooks Brown, a friend whom Harris warned off the premises just moments before he opened fire. "He loved the moment. He loved killing people, he liked that idea... That's how I knew it would end the way this did - kill all the hostages and then themselves. I couldn't see anything else."
The dead, many of whom were shot through the head at close range as they crouched in terror under desks and tables, included at least one teacher. Several survivors said Harris and Klebold laughed during the ordeal and spouted hate slogans against non-whites and sports players. One of the dead was black, the rest white.
The two boys were members of an on-campus group called the Trenchcoat Mafia, a withdrawn clique fascinated by the dark, satanic image of musicians such as Marilyn Manson. As well as trenchcoats, the group liked to wear black lipstick, paint their toenails and obsess about vampires, blood and death. Over the past year, Harris and Klebold grew their hair, took to wearing only black, and making vague threats to their classmates. "They were joking around saying, 'We are going to shoot you,'" said Alisa Owen, a fellow pupil.
Harris, in particular, was said to be obsessed with anything to do with Nazis or the Second World War and wore an armband saying "I hate people". He even took to spouting German in school corridors.
Although nobody on campus believed they could be serious, several warning signs appear to have been ignored or underestimated by the authorities. The Jefferson County Sheriff's Department said it had never heard of the Trenchcoat Mafia at Columbine, even though the group advertised itself in the 1998 high school yearbook with the legend: "Who says we're different? Insanity's healthy! Stay alive, stay different, stay crazy."
Nobody thought to link the group with the broader "goth" movement - the offshoot of punk obsessed with satanism and medieval torture - despite two alarming incidents in 1997. A 14-year-old boy whose room was daubed with swastikas and racist slogans stabbed his father in Lakewood, not far from the school. A few months later a senior at Columbine, himself a Trenchcoat member, shot his stepfather dead before taking his own life.
Eric Harris himself had shown several signs of potential violence. Brooks Brown's family say they complained to the police three times after Eric Harris posted a death threat against him on the Internet last year - a story not confirmed by the Sheriff's Department.
A class video he made featured him brandishing weaponry, presumably belonging to his father, Wayne, who retired recently from the military. Most alarming of all were a number of hate-filled Internet messages culminating in a posting on America On Line the day before the shootings that read: "Preparin' for the big April 20!! You'll all be sorry that day. Columbine High School sucks."
According to Carl Raschke, a professor at Denver University specialising in teenage cults, federal agents were on alert for some kind of attack on Monday 19 April but failed to act on the Internet warnings about the following day even though it was Hitler's birthday and a favourite date for neo-Nazis to commit acts of defiant violence.
"What these kids express is a cosmic statement against society," Professor Raschke said. "With their fascist chic they are clearly forwarding terrorist arguments. It's time to stop indulging in Pollyanna-ish belief that these kids are making some kind of fashion statement."
One of the big unknowns is whether Harris and Klebold acted alone or were given logistical support from outside. Their booby traps were sophisticated, and some were set with timers. And, unlike past school shootings in the United States, the whole operation appeared to be carefully planned. Police spokesmen said they were conducting hundreds of interviews but had found no evidence of a wider conspiracy yet.
The homes of the two boys were sealed off by police, and journalists were not able to talk to their parents. Both, however, lived in respectable neighbourhoods and did not arouse the suspicions of their neighbours, who described them as predominantly taciturn and difficult to get to know.
Their solidly middle-class background reflected the community as a whole, which found the tragedy particularly shocking because it imagined its school to be free of the usual urban blights of crime and dysfunction. Volunteers flooded into the area to provide counselling, food, spiritual solace and blood donations.
Offering his condolences yesterday, President Bill Clinton said: "All of us are struggling to understand exactly what happened and why."