The Cafeteria Bombs At Columbine High School

On the morning of April 20, 1999, Eric Harris and Dylan Klebold placed two 20lb propane tanks in the cafeteria (the “commons”) in duffel bags. Each propane tank was attached to a timer and a jug of gasoline with Visco Fuse (Cannon Fuse).

The timers were set to detonate at 11:17 a.m., which was precisely the time Eric had determined to have the largest number of students inside the cafeteria.

According to recovered diagrams, Eric determined that the cafeteria population would increase from roughly 300 to 500 people in the timespan of just 4 minutes – from 11:11 a.m. to 11:15 a.m.

Click the image below to view Eric’s drawing depicting the number of students in the cafeteria at specific times:

The overall plan was to create a massive explosion in the cafeteria to kill and injure as many people as possible, while they waited outside flanking the school’s main exits to shoot and kill fleeing survivors.

Construction And Purpose

Many people assume the cafeteria bombs were to be detonated remotely, since there was no detonator recovered at the scene. This is an understandable assumption, but is incorrect.

The type of explosion Eric and Dylan were trying to create requires no detonator and is called a BLEVE – or a “Boiling Liquid Expanding Vapor Explosion.” This type of explosion works by using fire to heat up the propane tank which causes the liquid propane to evaporate and rise to the top of the tank which builds up the pressure in the tank. The increase in pressure causes the propane tank’s safety valve to open and vent propane into the surrounding atmosphere. At this point, one of two things can happen:

Option 1: The pressure will continue to build beyond what the safety valve can safely vent, in which case the tank itself will explode, creating massive casualties.

Option 2: The fire will ignite the propane released by the safety valve causing smaller explosions, still with the potential for casualties but the damage would have been on a smaller scale.

Had the bombs exploded, the sides of the exploded tanks would have become deadly shrapnel for anyone remotely near the tanks.

Contrary to popular assumption, a propane tank exposed to fire that explodes is the exception to the rule. In other words, propane tanks aren’t as unstable as most people assume them to be.

A propane tank exploding after being exposed to fire is rare. There are hundreds of well documented cases of grass fires burning in people’s yards that directly burn the propane tanks for long periods of time and they never explode.

Clocks With Plastic Hands?

A common question that comes up around the construction of the cafeteria bombs is the use of Westclox manual alarm clocks with plastic hands and stationery bells that don’t touch when they ring. Most people have a general understanding that plastic does not conduct electricity, and therefore wonder if the use of plastic hands is what prevented the bombs from detonating. Additionally, most people believe that the bells need to ring together to complete the circuit of a bomb made with an alarm clock. This belief is absolutely false.

When constructing the cafeteria bombs, Eric and Dylan did not use the plastic hands to complete the circuit, they used the bells which were made of metal, and the circuit would have been completed on the inside of the device – not the outside. Because the circuit is completed based on the manipulated wiring inside the alarm clock, the bells do not need to touch – they only need to ring.

This is very basic circuitry that leverages the natural arc of electricity when the inside wiring of the alarm clock is manipulated in a simple, specific way. All it takes to ignite flammable liquid (like gasoline) in this manner is a tiny spark. For obvious reasons, we’re not going to provide further details on this process.

Why Did The Bombs Not Detonate?

It’s possible that the alarms failed to go off due to being stuffed into duffel bags, or being turned on their sides. Anything could have potentially pushed the alarm buttons back in. It’s also possible that the inside wiring was not created correctly, however, given the simplicity of creating such a basic circuit, it’s not likely the wiring was the problem. A 6 year old could create this kind of a circuit on the first try. In fact, high school students routinely create this kind of circuit in science class projects with minimal instruction.

Also, they created a diversionary time bomb which was constructed similarly save for a smaller propane tank, and that bomb successfully detonated, although the fire was extinguished before the pressure in the tanks could cause the secondary, intended explosion.

Would The Bombs Have Produced Massive Casualties?

Had the timers gone off as planned to ignite the gasoline, Eric and Dylan would have failed to produce massive casualties from these bombs. Allow me to explain why:

Problem #1: Propane Tanks DO NOT Explode Easily

Believe it or not, a propane tank does not explode easily. In fact, it takes a lot of effort to get one to explode. You could run over a propane tank with your car or fly a commercial plane into it and the tank would not be likely to explode. Similarly, shooting a propane tank will not necessarily cause it to explode.

In the Columbine cafeteria video footage, Eric Harris is seen kneeling on the stairs as he shoots over the railing at one of the propane tanks with his 9mm rifle. The tank still refuses to explode.

Propane tanks have a built-in pressure release valve that automatically opens when the pressure inside the tank reaches a specific level. Most propane tanks are built to withstand up to 250 PSI, and, when filled and maintaining a temperature of 70 degrees Fahrenheit, the pressure in the tank is only about 110 PSI. This is because tanks, as a rule, are filled only to 80% capacity. So while a 20 pound propane tank has a 4.2 gallon capacity, it is never filled to that capacity, therefore creating even more of a safety net inside the tank.

A BLEVE occurs when the pressure that builds up in the tank greatly exceeds what the built-in safety relief valve can vent into the surrounding atmosphere. If the valve can’t keep up with the rising pressure, the tank will eventually explode.

A propane tank BLEVE is not an instantaneous explosion. It takes time for the fire to burn next to the propane tank in order to heat the contents and force the tank to build pressure and eventually explode. This is by no means an easy feat.

Problem #2: The Sprinkler System

If the timers had successfully ignited a fire, the school’s sprinkler system would have put out the fire long before the tanks could build up enough pressure to trigger the safety release valve to open.

Problem #3: Schools Are Trained To Evacuate

Schools are trained to evacuate students in around 90 seconds when the fire alarm goes off. Had the fire ignited on time, the fire alarm would have gone off at 11:17, and the students would have fled within minutes. By the time the propane tanks would have exceeded their pressure limits (if at all), the cafeteria would have been empty.


Making bombs is not only illegal, but very dangerous. Do not EVER try to recreate Eric and Dylan’s recipes and strategies for explosives. They used unstable materials and haphazard construction and were very lucky they didn’t blow themselves up. A lot of instructions for making bombs on the internet purposely instruct you to do things that will result in injury and/or death. Don’t be an idiot!