On April 20, 1999, two students at Columbine High School in Littleton, Colo. – Eric Harris and Dylan Klebold – killed 12 students, a teacher and themselves. The violence they carried out seems as unbelievable today as it was nearly 15 years ago — when the term Columbine became forever linked to deadly school violence.

Shortly before 11:30 a.m., the pair, wearing trench coats, walked up to the 2,000-student school, and began shooting before they even got inside. A detailed chronology, as well as information about Harris and Klebold, can be found in this report by the Columbine Review Commission in Colorado.

Among the reports findings:

- Harris and Klebold were “disgruntled seniors at the school ... determined to kill as many teachers and fellow students as possible.”

- As for the upbringing and background of Harris and Klebold, the report stated: “Both sets of parents reported normal upbringings for their sons, including their participation in scouting and little league baseball and soccer leagues. As they grew older, both boys became interested in computers.”

- In a November, 1997, entry in his journal, Klebold had said he wanted to go on a killing spree. Harris expressed a more detailed written record on a personal Internet page, according to the report, where he wrote of bomb-making and people he would like to kill. “I hate you people for leaving me out of so many fun things,” Harris wrote, according to the report.

- The report also explored how warning signs were missed. “There were indications that the pair had suicidal and violent tendencies. However, they managed successfully to mask their true intentions from their parents and school administrators and perhaps from the bulk of their fellow students,” the report stated.

- The two students spent a year planning the attack and gathering explosives and weapons. They planted two 20-pound propane bombs in the school cafeteria, planning on shooting fleeing survivors after the bombs erupted. But the bombs never ignited.

- The two then marched toward the school anyway, first killing a student who was eating lunch on the grass. “When they stormed the school,” the report stated, “Klebold and Harris were armed with dozens of explosive devices of varying potency, seven knives, two illegal sawed-off shotguns, a 9 mm semiautomatic assault pistol and a 9 mm semiautomatic carbine rifle, as well as a substantial supply of ammunition ... They then entered the school building and began firing at students leaving the school cafeteria, wounding five students and deliberately executing one of them.”

- About 11:30 a.m., Harris and Klebold entered the school library, where 56 students, two teachers and two library employees had taken refuge, according to the report. Most of the students tried to take cover under tables. “Witnesses reported hearing the two demand that everyone get up, or that ‘jocks’ with white hats stand up. No one responded to the command, so one of the gunmen said, ‘Fine, I’ll start shooting,’” the report stated.

- After killing victims in the library, “Klebold and Harris continued to wander through the school hallways, igniting more pipe bombs,” the reports stated.

- The end came just after noon. “The two left the cafeteria a final time and returned to the upstairs library; they moved to the library windows from where they exchanged gunfire with law enforcement officers from several agencies who were covering paramedics while they rescued students wounded outside the school. Investigators believe that at approximately 12:08 pm, Harris and Klebold turned their weapons on themselves and committed suicide.”

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