In addition to the shootings, the complex and highly planned attack involved a fire bomb to divert firefighters, propane tanks converted to bombs placed in the cafeteria, 99 explosive devices, and carbombs.
The perpetrators, senior students Eric Harris and Dylan Klebold, murdered 12 students and one teacher. They injured 21 additional people, and three more were injured while attempting to escape the school. The pair subsequently committed suicide.
Although their precise motives remain unclear, the personal journals of the perpetrators document that they wished their actions to rival the Oklahoma City bombing and other deadly incidents in the United States in the 1990s. The attack has been referred to by USA Today as a "suicidal attack [that was] planned as a grand—if badly implemented—terrorist bombing." The massacre has been reported as "the deadliest high school shooting in US history."
The massacre sparked debate over gun control laws, high school cliques, subcultures, and bullying. It resulted in an increased emphasis on school security with zero tolerance policies, and a moral panic over goth culture, gun culture, social outcasts (even though the perpetrators were not considered outcasts), the use of pharmaceutical anti-depressants by teenagers, teenage Internet use, and violence in video games.
On the fifth anniversary of Columbine, the FBI's lead Columbine investigator and several psychiatrists published their conclusions in a news article. These conclusions stated Harris was a clinical psychopath, whereas Klebold was depressive. Harris had been the mastermind, having a messianic-level superiority complex, and had hoped to demonstrate his superiority to the world, whereas Klebold, having repeatedly documented his desires to commit suicide in his diaries—particularly due to his lack of success with women—had primarily participated in the massacre as a means to simply end his life. Dr. Dwayne Fuselier, the supervisor in charge of the Columbine investigation, would later remark: "I believe Eric went to the school to kill and didn't care if he died, while Dylan wanted to die and didn't care if others died as well." Klebold's final remark in the videotape he and Harris made shortly before their attack upon Columbine High School is a resigned statement made as he glances away from the camera: "Just know I'm going to a better place. I didn't like life too much."
The attack was the culmination of more than a year of planning, firearms acquisition, and bomb building. Harris's journals, in particular, show methodical preparation over a long period of time, including several experimental bomb detonations. By comparison, the journals Klebold populated initially contained few references to violence (although from January 1999 onward references to violence would become more frequent). By far the most prevalent theme in Klebold's journals is his private despair at his lack of success with women, which he refers to as an "infinite sadness".
For prior behavioral issues, Harris had been prescribed the SSRI antidepressant Fluvoxamine; toxicology reports confirmed that Harris had Fluvoxamine in his bloodstream at the time of the shootings. Klebold had no medications in his system.
Both Harris and Klebold were fans of video games such as Doom, Wolfenstein 3D, and Duke Nukem. Harris often created levels for Doom that were widely distributed; these can still be found on the Internet as the Harris levels. Rumors that the layout of these levels resembled that of Columbine High School circulated, but appear to be untrue. Harris spent a great deal of time creating another large mod, named Tier, calling it his "life's work." The mod was uploaded to the Columbine school computer and to AOL shortly before the attack, but appears to have been lost. One researcher argued that it is almost certain the Tier mod included a mock-up of Columbine High School.
Parents of some of the victims filed several unsuccessful lawsuits against video game manufacturers. Harris and Klebold were fans of the movie Natural Born Killers, and used the film's acronym, NBK, as a code in their home videos and journals.
The Columbine shootings influenced subsequent school shootings.
In 2009 sociologist Ralph Larkin of the John Jay College of Criminal Justice at City University of New York wrote that Harris and Klebold established a script for subsequent school shootings. Larkin examined the twelve major school shootings in the U. S. in the eight years after Columbine and found that in eight of those "the shooters made explicit reference to Harris and Klebold." Larkin concluded:
In 2012 sociologist Nathalie E. Paton of the National Center for Scientific Research in Paris analyzed the online videos created by post-Columbine school shooting perpetrators. A recurring set of motifs was found, including self-portraits with firearms in which the perpetrator points his gun "at the camera, then at his own temple, and then spreads his arms wide with a gun in each hand; the closeup" and "the wave goodbye at the end," as well as explicit statements of admiration and identification with previous perpetrators. Paton said the videos serve the perpetrators by distinguishing themselves from their classmates and associating themselves with the previous perpetrators.
A 2014 investigation by ABC News identified "at least 17 attacks and another 36 alleged plots or serious threats against schools since the assault on Columbine High School that can be tied to the 1999 massacre." Ties identified by ABC News included online research by the perpetrators into the Columbine shooting, clipping news coverage and images of Columbine, explicit statements of admiration of Harris and Klebold, such as writings in journals and on social media, in video posts, and in police interviews, timing planned to an anniversary of Columbine, plans to exceed the Columbine victim counts, and other ties.
A 2015 investigation by CNN identified "more than 40 people...charged with Columbine-style plots." According to psychiatrist E. Fuller Torrey of the Treatment Advocacy Center, a legacy of the Columbine shootings is its "allure to disaffected youth."
In 2015 journalist Malcolm Gladwell writing in The New Yorker magazine proposed a threshold model of school shootings in which Harris and Klebold were the triggering actors in "a slow-motion, ever-evolving riot, in which each new participant's action makes sense in reaction to and in combination with those who came before."