The official story
According to the official story, Eric Harris and Dylan Klebold shot and killed 12 students and one teacher at Columbine High School on April 20, 1999. According to the official story, they acted alone and nobody else was involved.
Below is a summary of the official story. This summary is brief because the majority of the official story is demonstrably false, and to learn about what actually happened, please reference this timeline based on witness accounts and investigative documents and review the discrepancies in the official story. Don't take all of the information on this page as factual as most of it is easily disproven.
General overview of the official story:
On the morning of April 20, 1999, neither Eric nor Dylan showed up to their early morning bowling class. Instead, they were busy getting ready for the attack they were about to perpetrate against Columbine High School.
It's said they skipped the rest of their morning classes and at 8:36 a.m., Eric bought 2 propane tanks at a local gas station and bought another propane tank at a different gas station at 9:12 a.m.
At 8:41 a.m., Eric and Dylan were at Eric's house picking up bags full of gear and bombs, filmed a brief last segment on the "Basement Tapes," and that's when Eric posted a copy of his AOL profile into an AOL chatroom.
Then they went back to the high school and nobody knows what they did for the next couple of hours, but a few people spotted them in random places around the school. It's said they waited for the custodian to change the cafeteria surveillance tape before placing the two large propane bombs in the cafeteria so they wouldn't get caught on tape. It's said that before the tape cuts at 11:14 a.m., the bags are not visible, but when the tape resumes at 11:22 a.m., the bags are visible.
At some point, they went to Clement Park to gear up and drove back to the school. When Eric parked his car, he ran into Brooks Brown and told him to "go home" stating that he liked him now, despite their rocky relationship.
The cafeteria bombs were set to explode at 11:17 a.m. and they hoped to kill about 500 people in the cafeteria and flush survivors out so they could shoot them. When the bombs didn't explode, they just started shooting at kids outside.
At 11:24 a.m., teacher Dave Sanders ran up the stairs and was shot twice from behind. He fell to the floor and a teacher helped him into a classroom where he lay bleeding for hours. SWAT found him at 2:42 p.m and called for medical assistance. When paramedics got to him, he was alive, but not savable so they moved him into a storage room where he died.
At 11:26 a.m., Columbine's School Resource Officer (SRO) Deputy Neil Gardner arrived in the parking lot after hearing about a girl down over the radio while parked in Clement Park monitoring the smoker's pit. He had a shootout with Eric from 60 yards away in the parking lot; Eric was standing in front of the West doors.
After killing two people outside and wounding several others, the shooters retreated into the school's library, where they taunted and killed ten more students before committing suicide at 12:08 p.m.
They were armed with four firearms, several knives, and various types of homemade explosives.
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Additional resources:
Discrepancies in the official story
Timeline of events according to witnesses and investigators