Stunned by the rampage of violence and death at Columbine High School, students, parents and teachers unable to sleep or to sit fretting at home flocked to their churches for a long night of special services, prayers, hugs and tears.
The 1,100-seat St Francis Cabrini Catholic Church, just a few hundred yards from the school, was overflowing. The singers could barely get through their lines without their voices cracking. And, as five high school students told their stories, the congregation could barely contain its sense of shock and grief.
"You know, you don't really know what blind fear is until it rolls up your stomach and asphyxiates you, chokes you," said one of the five, a 16-year-old girl who was in music practice when the shooting started. "When we were told there was shooting, a titter went round the room. We thought - no, this is Littleton... this doesn't happen here."
Indeed, this affluent suburb south-west of Denver seems like the last place that should be afflicted by this kind of tragedy. One of the newer suburbs in this fast-growing city, its well-appointed housing estates look the very model of conservative, middle-class contentedness and civic cohesion.
The 1,800-student Columbine High School seemed to be a thriving community, with few of the drug or gang problems that afflict inner-city schools. As soon as the shooting started, many grief counsellors and social workers turned up to offer their services.
Parents were shepherded to the Leawood Elementary School a few hundred yards away, where an impromptu information service posted news of survivors and students taken to hospital. Despite the high emotion - including several people rushing outside to vomit from fear and shock - parents were encouraged to provide physical descriptions and dental records of their children to keep the agony of waiting as short as possible.
Around the Denver area, about 2,000 people queued to donate blood, overwhelming blood bank staff who urged them to come back over the next few days and weeks because they could not cope with the rush of offers.
The local baseball teams, the Colorado Rockies and the Denver Nuggets, spontaneously cancelled their games on Tuesday night.
Such solidarity only heightened the incomprehension. In their homilies, church ministers could only express their awe in the face of evil, without any obvious explanations. "We are safe nowhere," said Jerry Rohr, the priest at the Light of the World Catholic church in Littleton. "Kosovo is in our back yard. It is not some place far away."
And yet, after half a dozen school shootings around the United States in the past two years, a pattern does begin to emerge. Nearly all the schools affected, from Pearl, Mississippi to Springfield, Oregon are in well-to-do suburbs or small towns with predominantly white populations.
In every case, a disaffected, alienated teenager or teenagers had access to terrifying weaponry thanks to the gun culture that flourishes in such places. In the US, the labels "conservative" and "Christian" often go together with a robust pioneer spirit and large domestic arsenals of hunting rifles, semi-automatic weap-ons and more.
The decline of big-city centres and the proliferation of suburban housing developments in nearby open country has brought that pioneer spirit much closer to large population centres. Denver is a perfect case in point, since it is one of the fast-growing metropolitan areas in the country, thanks to its pleasant climate and proximity to the Rockies.
The spacious housing developments of Littleton and the surrounding Jefferson County are the ideal environment for a disturbed teenager to act out his fantasies without attracting much attention. The community is new and imperfectly formed, and neighbours neither know each other nor have much need for contact beyond a wave each morning from the windows of their four-wheel-drive cars.
Even guns would be unlikely to attract much attention, since most Jefferson County residents have plenty of them and are among the voters supporting attempts by Colorado's Republican governor, Bill Owens, to ease restrictions on concealed weapons and other gun-control laws.
Neighbours of the two main suspects, Eric Harris and Dylan Klebold, said repeatedly yesterday that they barely knew the boys and had no reason to think anything was wrong - exactly the response given by Kip Kinkel's neighbours in Springfield, Oregon after he killed his parents and opened fire in his high school cafeteria last May.
What makes America's school shootings so frightening is that the danger is clearly there, but more often than not deeply hidden.
"There are hundreds of kids like this across America," warned Carl Raschke, a religious studies professor at Denver University and a specialist in teenage cult violence. "This is going to happen again."