Within days of the massacre in April 1999, 15 temporary six-foot crosses were put up in Clement Park near Columbine high school in honour of the 12 students and a teacher slain in the massacre, and the two teenage killers, Eric Harris, 17, and Dylan Klebold, 18, who had attacked their school armed with bombs and guns. In this picture the killers' crosses have been covered. Two years later, only 13 crosses stood in their place.
In March this year, students at Santana high school in California were remembering a classmate killed in a shooting there, by wearing crosses and peace ribbons on their shirts; the victim's mother accepted his diploma at a sombre graduation ceremony.
Others found more disturbing ways to mark the anniversaries. Two students in Las Vegas were suspended after threatening to shoot up their schools. "They made verbal threats to come to school today and do a Columbine-style shooting," a police official said. Two schools in New York were closed after receiving threatening email messages, and students at a school in Florida were sent home when authorities found what turned out to be a fake bomb.
While Americans are good at remembering these tragedies, they have failed to reach consensus on why they happen and how to avoid more. And with a Republican president sympathetic to gun owners, there is little likelihood that te contentious issue of gun control will be resolved. But there have been some changes. The providers of a gun used in the Columbine killings have been forced to pay part of a $2.5-million (pound;1.8-million) settlement to the victims' families.
Tom Mauser, the father of one of the students killed at Columbine, helped form a gun-control group that successfully lobbied for a law in Colorado requiring criminal background checks on buyers at gun shows. But so frustrated has Mr Mauser become with the federal government's resistance to closing this loophole at national level, he switched his party affiliation to Democrat, and picketed the headquarters of the pro-gun National Rifle Association carrying a picture of his son, Daniel, on a placard. The NRA has fought efforts to ban assault weapons, including the kind used by the Columbine killers. When Mr Mauser refused to leave, he was arrested.
One interest group conservatives have been willing to attack is the entertainment industry, even though a US surgeon general's report this year found that television, movies, and video games play a minor role in contributing to youth violence. Shock-rocker Marilyn Manson, whose music was reportedly a favourite of the trench-coated Columbine killers, has been asked to cancel an appearance near the school this summer. But the reason these boys turned killers - and so many have tried to copy them - remains a mystery.
Photograph Steve Peterson
www.danielmauser.com www.nra.org www.memoriallink.comcolumbine
www.bradycampaign.org(Handgun Control Incorporated, a national gun-control group) www.surgeongeneral.govlibraryyouthviolence