On April 20, 1999, two students at Columbine High School in Littleton, Colorado shot and killed twelve students, one teacher and wounded twenty others before turning their guns on themselves. It became the deadliest high school shooting in American history. Senior students Dylan Klebold and Eric Harris arrived at school that morning wearing black trench coats containing hidden weapons, including an assault rifle, sawn-off shotguns, handguns, and homemade grenades. The pair began their assault at approximately 11.15a.m by randomly shooting at students in the parking lot before proceeding to enter the main school building. Once inside, they headed to the cafeteria where they detonated a homemade pipe bomb and fired at nearby students. In the ensuing minutes, the killers attacked the high school's choir room, auditorium, gymnasium, and the library, where the majority of the killings took place.
Police arrived approximately twenty minutes after the incident had begun. Upon arrival they found several explosive devices positioned around the school building. At 12 p.m, ambulances began transporting wounded students to hospital as the school's parking lot filled with SWAT and bomb-disposal teams and the emergency services. Half an hour later, the shooting stopped. Klebold and Harris had committed suicide in the library, although their bodies were not located until 4.30p.m that afternoon. It later transpired Kelbold and Harris had left detailed journals and video diaries in which they described their ambition to create an incident to rival the 1995 Oklahoma bombing. Both of the killers were bullied at Columbine and it appeared they were exacting an extreme revenge. They both also suffered degrees of mental illness, including depression and anger management issues that may have contributed to their state of mind.
In the days after the shootings, Littleton became the center of the nation's attention. A memorial service held at a nearby shopping center five days after drew an estimated 70,000 people. Spontaneous memorials and messages from all over the country began to appear in Clement Park by the school. Some 200,000 people visited the park before the memorials were dismantled. An estimated 150 counselors were on the scene in the early days, and more than 1,600 people received counseling from the Jefferson Center for Mental Health and its hastily formed adjunct agency, Columbine Connections. A permanent memorial to honor and remember the victims of the shootings was unveiled in Clement Park on September 21, 2007.
After Columbine, the focus across the country turned to how parents and schools could help prevent similar tragedy in future, with steps being taken to provide better communication and counseling for students. The shootings provoked debates regarding gun control laws, the availability of firearms in the United States, and youth gun violence. These discussions also centered on the nature of high school cliques, subcultures, bullying, and the role of violent movies, music and video games in American society. The shootings resulted in an increased emphasis on school security. A campaign headed by Tom Mauser, father of a murdered student, led to four bills being passed by the Colorado legislature in order to control handgun acces
In some pro-gun states, bills were introduced to regulate gun shows. Billionaire Andrew J. McKelvey created Americans for Gun Safety, which funded referenda in Oregon and Colorado; the idea being to create a requirement that all persons purchasing handguns must first pass stringent criminal background checks. In the weeks and months after Columbine, schools across the United States began to enforce new security measures such as security guards, metal detectors and see-through backpacks. This is known as the Columbine effect. Many schools implemented school door numbering to improve public safety, while several schools ordered their students to wear computer-generated identification. In addition, police departments reassessed their tactics ahead of future similar situations.
Columbine witnessed further fallout in the months after the shooting when the mother of a 17-year-old girl victim of the massacre committed suicide. Another student killed himself shortly after the first anniversary. A prominent local minister resigned his Lutheran congregation sixteen months after the shooting, citing stress over the incident and controversy within the congregation over his outreach to the Klebold family. Michael Moore's 2002 documentary "Bowling for Columbine" explored possible causes of the massacre. In 2009, New York Times contributor Dave Cullen published "Columbine" a bestselling book examining the shootings.