There has been much public speculation that the bombs were brought into the school several days before April 20, possibly during the "after-prom" party held in the school the prior weekend. But the cafeteria videotape of April 20 tells a different story.

Each school day, a videotape records the activities in the school cafeteria. Four separate cameras, catching images from four different angles, are set up in corners of the cafeteria. Their primary purpose is to capture on tape any general mischief, food fights or other problems that can happen when 500 teenagers gather in one location. Because of the cameras’ locations in the cafeteria, there are a few blind corners that the cameras cannot "see" or record on tape.

The first tape of each day begins recording scenes around 7 to 8 a.m. For the most part, the cafeteria is empty in those first few hours except for a scattering of students taking advantage of the quiet study area during a free class period. The morning tape is replaced by a second around 10:30 or 11 a.m., shortly before the "A" lunch period begins at 11:15 a.m. and the activity in the cafeteria intensifies.

On Tuesday, April 20, the building custodian was behind schedule and was late changing the cafeteria videotape. At 11:14 a.m., he unlocked the door to the backroom where the recording VCR is set up. He hit the eject button, removed the morning videotape and inserted a new one. But the videotape to record both of the day’s lunch periods and on into the afternoon was one that was being recycled. It had to be rewound. While waiting for the tape to rewind, the custodian left the room to make a quick call home. He hit "record" on tape #2 at 11:22.

In the time between 11:14 when the custodian stopped the first videotape and ejected it from the VCR, and 11:22 when the second tape began to record lunchtime activities in the cafeteria, two large duffel/gym bags appeared beside two separate tables in the lunch room. In the eight minutes it took the custodian to eject the first videotape, rewind the second one and make a short phone call, the bags became visible.

The cafeteria tapes were sent to the Federal Bureau of Investigation (FBI) lab in Quantico, Virginia, where they were enlarged and enhanced several times. The lab examined both tapes frame by frame. The quality of the tapes, even when enhanced, was poor. But lab experts are confident that in the first frames of the new videotape beginning at 11:22, two large shapes can be seen on the floor in spots that were empty at 11:14.

The building custodian noticed nothing out of the ordinary prior to the first lunch period. Besides changing the cafeteria videotapes, his daily routine included walking through the cafeteria after each class period looking for items left behind by students using the period to study. He felt certain that he would have noticed any large duffel bags or backpacks left in the area before lunch began.

(Note: A glimpse into the mindset of Klebold and Harris became clearer as investigators realized the extremely short period of time the suspects allowed between the time the bombs were placed in the cafeteria and the time set for them to explode – basically three minutes. Since the cafeteria videotape shows that the bombs were not beside the tables at 11:14 and the bombs were set to detonate at 11:17, little leeway was allowed for the two suspects to escape from the fireball they had planned. ) 

Investigators established that Harris and Klebold brought the bags containing the large propane bombs into the cafeteria and set them beside two cafeteria tables at the beginning of the first lunch period. The bombs were hidden in duffel bags so they easily blended in with the 400-plus backpacks strewn on the floor, under tables and chairs throughout the cafeteria. Most high school students carry some type of backpack. Two more bags brought in by one of their peers would not raise anyone’s suspicions.

Nearly 500 students were in the cafeteria that day. Investigators interviewed all of them at least once, but no one actually recalled either of the gunmen walking in with a duffel bag and setting them down near tables (PP and QQ) where the two normally sat with friends at lunch.

The bomb technicians discovered that the bombs were equipped with timers set to detonate at 11:17 a.m.

In some of Harris’ writing, investigators found references to the optimum times during the school day and the location to detonate the bombs in order to kill the greatest number of people. Those notes appear on pieces of scrap paper, on corners of his daybook, scribbly little notes written as he sat in the school cafeteria taking a count of the number of students in that location at a specific time. The bell announcing the end of class and the passing period for the next class period or "A" lunch rang at 11:10 a.m.

  • At 11:10, Harris estimated 270 to 300 people in the cafeteria and makes a note of "heavy additions" and the "lines start." 
  • At 11:11, he counted about 300 to 350 people in the cafeteria, 
  • At 11:12 to 11:13 between 350 and 450. He noted that at 11:14 to 11:15 there were over 500 students.

Klebold’s last entry in his school notebook gave a chilling timeline for April 20. "Walk in, set bombs at 11:09 for 11:17. Leave..." The two apparently determined that 11:17 a.m. was the most opportune time to cause the most damage and deaths. The timers on the propane bombs were set for 11:17.

Sometime before 11:17, Harris and Klebold placed the two propane bombs in the cafeteria, and went back out to the student parking lots to their respective cars. The belief is that they then would shoot any surviving students who were able to escape the fireball. Bombs in their cars were set to explode after those in the cafeteria.