Did you read the article? "Upon their arrival at Columbine, the duo met near Harris's car and armed two 20 pound (9 kg) propane bombs before entering the cafeteria a few minutes prior to the beginning of the A lunch shift, placing the duffel bags containing the bombs inside. Each bomb was set to explode at approximately 11:17 a.m.[4] However, a custodian removed the security camera video tape, rewound it, and placed a new tape into the slot just as the duo entered the cafeteria. Though the act of placing the bombs was not recorded, just as the new tape was started, the bags were clearly visible. Klebold and Harris then returned to their vehicles to await the explosion of the bombs they had planted."
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Did you read the article? "Upon their arrival at Columbine, the duo met near Harris's car and armed two 20 pound (9 kg) propane bombs before entering the cafeteria a few minutes prior to the beginning of the A lunch shift, placing the duffel bags containing the bombs inside. Each bomb was set to explode at approximately 11:17 a.m.[4] However, a custodian removed the security camera video tape, rewound it, and placed a new tape into the slot just as the duo entered the cafeteria. Though the act of placing the bombs was not recorded, just as the new tape was started, the bags were clearly visible. Klebold and Harris then returned to their vehicles to await the explosion of the bombs they had planted."