Case overview
On April 20, 1999, two students carried out a planned attack at Columbine High School in Littleton, Colorado, killing 12 students and one teacher before taking their own lives. The 49-minute assault involved firearms, explosives, and a series of failures in the devices the shooters had planted throughout the school. The attack ended the deadliest high school shooting in US history at the time and triggered national debates about school safety, gun access, and warning signs that investigators later documented in journals, videos, and witness statements.
The planning phase
Eric Harris, 18, and Dylan Klebold, 17, began planning the attack more than a year in advance. Investigators recovered journals, video recordings, and written materials that detailed their intentions. Harris kept a diary in which he outlined his desire to cause mass casualties and expressed contempt for humanity. Klebold’s writings reflected suicidal ideation and a desire for violence, though his motivations appeared less defined than Harris’s.
The two acquired firearms through straw purchases facilitated by friends. They purchased a TEC-9 semi-automatic handgun, a Hi-Point 9mm carbine, and two shotguns. They also built nearly 100 explosive devices, including pipe bombs, propane tank bombs, and carbon dioxide cartridge bombs. Many of these devices were intended to detonate during the attack but failed to function as planned.
In the months before the massacre, both students recorded a series of videos later referred to as the “Basement Tapes.” In these recordings, they discussed their plans, expressed anger toward classmates, and made references to previous violent incidents. Law enforcement reviewed the tapes during the investigation, but the Jefferson County Sheriff’s Office withheld them from public release and later destroyed them in 2011.
The attack begins
At 11:10 a.m. on April 20, Harris and Klebold placed two duffel bags containing propane bombs in the school cafeteria. The bombs were set to detonate at 11:17 a.m., during the lunch period when the cafeteria was most crowded. Investigators later estimated that if the bombs had functioned, they could have killed or injured hundreds of students.
When the bombs failed to detonate, Harris and Klebold proceeded to the school’s exterior. At 11:19 a.m., they began shooting near the west entrance. Their first victim was Rachel Scott, a 17-year-old student who was eating lunch outside with a friend. Richard Castaldo, seated with Scott, was shot multiple times but survived.
Over the next three minutes, the shooters moved across the exterior of the school, firing at students. Daniel Rohrbough, Sean Graves, and Lance Kirklin were shot on the west staircase. Rohrbough died at the scene. Michael Johnson was struck while attempting to flee. Mark Taylor and Anne-Marie Hochhalter were shot on the lawn outside the cafeteria. Teacher William “Dave” Sanders, hearing the gunshots, began directing students to safety inside the building.
Inside the school
At 11:24 a.m., Harris and Klebold entered the school through the west entrance. They threw pipe bombs down the main hallway and into the cafeteria. Most of the explosives either failed to detonate or caused minimal damage. The shooters then moved toward the library, located on the second floor.
At 11:29 a.m., they entered the library, where 52 students, two teachers, and two librarians were hiding under tables. Over the next seven and a half minutes, the shooters killed 10 people and injured 12 others. The victims in the library were Kyle Velasquez, Steven Curnow, Cassie Bernall, Isaiah Shoels, Matthew Kechter, Lauren Townsend, John Tomlin, Kelly Fleming, Daniel Mauser, and Corey DePooter.
Witness accounts from the library varied in detail, but several survivors reported that the shooters taunted victims, made references to targeting specific students, and reloaded weapons multiple times. At one point, Harris looked out the library windows and fired at police officers and fleeing students outside. The shooters left the library at 11:36 a.m.
The final movements
After leaving the library, Harris and Klebold wandered through the school’s hallways and cafeteria. They attempted to detonate the propane bombs they had placed earlier by shooting at them, but the devices did not explode. Video footage from cafeteria security cameras captured the two moving through the space, throwing small explosives, and firing sporadically.
At 11:44 a.m., they returned to the cafeteria and attempted again to ignite the propane tanks. They then walked upstairs toward the science area, threw a Molotov cocktail into an empty storage room, and set off the fire alarm. They briefly interacted with students hiding in classrooms but did not attempt to enter locked rooms.
At 12:02 p.m., Harris and Klebold returned to the library. By this time, all surviving students and staff had fled. The two moved to the west windows, fired several shots outside, then walked to the center of the room. At 12:08 p.m., both died by suicide. Harris shot himself in the mouth with a shotgun. Klebold shot himself in the left temple with a TEC-9.
The teacher who bled out
Dave Sanders, a business and computer teacher, was shot twice in the torso while helping students evacuate. He made his way to a science classroom on the second floor, where students and a teacher barricaded the door and attempted to provide first aid. They used shirts to apply pressure to his wounds and placed a sign in the window reading “1 Bleeding to Death.”
SWAT teams did not reach Sanders for more than three hours. He died at 2:56 p.m., approximately four hours after being shot. The delay became a focal point of criticism directed at law enforcement’s response. The Jefferson County Sheriff’s Office defended the decision to secure the building methodically, citing concerns about additional explosives and the uncertainty of whether the shooters were still active.
Law enforcement response
The first deputy arrived on scene at 11:24 a.m., five minutes after the shooting began. Within minutes, officers exchanged gunfire with Harris outside the building. Additional units from multiple agencies converged on the school, but coordination was hampered by communication failures and confusion about the shooters’ locations.
SWAT teams did not enter the building until approximately 12:00 p.m. They moved room by room, evacuating students and searching for suspects and explosives. The library, where the bodies of Harris and Klebold were located, was not secured until 3:22 p.m. The last survivors were evacuated shortly thereafter.
The response protocol in place at the time prioritized containment and negotiation. Officers were trained to establish a perimeter and wait for specialized units. The Columbine attack exposed the inadequacy of this approach during an active shooting, leading to widespread changes in law enforcement tactics. Most agencies now train officers to immediately engage an active shooter rather than wait for SWAT.
The investigation and disputed evidence
In the months following the attack, investigators conducted more than 10,000 interviews and processed thousands of pieces of evidence. The Jefferson County Sheriff’s Office released a final report in May 2000, but questions persisted about what authorities knew in advance and whether warning signs were ignored.
In 1998, a year before the massacre, the parents of Brooks Brown filed a police report alleging that Eric Harris had made death threats and maintained a website containing violent writings. A sheriff’s deputy drafted an affidavit for a search warrant targeting Harris’s home, but the warrant was never executed. The existence of the draft affidavit was not disclosed until 2001, prompting accusations of a cover-up and resulting in lawsuits against the sheriff’s office.
Another disputed detail involved witness reports that more than two shooters were involved. Several students reported seeing individuals they did not recognize, and some described coordinated movements that seemed inconsistent with only two attackers. Law enforcement investigated these claims and concluded that all shooting was carried out solely by Harris and Klebold. Confusion during the attack, including misidentifications and distorted perceptions under extreme stress, likely contributed to conflicting witness statements.
The question of motive has never been fully resolved. Investigators found no evidence that Harris and Klebold specifically targeted students based on religion, race, or social group, despite early reports suggesting otherwise. The widely circulated claim that Cassie Bernall was killed after affirming her belief in God was later determined to be inaccurate. That exchange occurred with another student, Valeen Schnurr, who survived. Harris’s writings suggested he was driven by a desire for notoriety and a belief in his own superiority. Klebold’s motivations appeared more tied to depression and a desire to follow Harris.
Aftermath and influence
The Columbine massacre reshaped school safety policy across the United States. Schools implemented lockdown drills, installed surveillance systems, and increased security personnel. Threat assessment protocols were developed to identify students exhibiting warning signs. The attack also intensified debates over gun control, media violence, and adolescent mental health.
The shooting has been referenced or cited as inspiration by numerous subsequent attackers. The phenomenon of individuals fixated on Harris and Klebold, sometimes referred to as “Columbiners,” has complicated efforts to prevent copycat incidents. Security agencies continue to monitor online communities where the attack is glorified.
Families of the victims filed multiple lawsuits against the families of the shooters, the school district, and the sheriff’s office. Most were settled out of court. The families of Harris and Klebold issued public apologies but have largely remained out of the public eye.
Where to look next
- Documentary: “Columbine: Understanding Why” (CNN)
- Documentary: “The Columbine Massacre” (Investigation Discovery)
- Book: “Columbine” by Dave Cullen
- Book: “A Mother’s Reckoning: Living in the Aftermath of Tragedy” by Sue Klebold
- Podcast: “The Prosecutors” (Brett Talley and Alice LaCour)
