NEW YORK CITY- A vehicle borne explosive device comprised of propane tanks, fertilizer, fireworks, and gasoline failed to detonate inside an old SUV, which had been parked in Times Square on the night of May 1.

The putative bomber, Faisal Shahzad, has been tracked down, arrested, and charged with a number of crimes including attempting to commit an act of terror and attempting to use a weapon of mass destruction.

But there are still questions on the minds of those who work with IEDs. How was this device supposed to work, and why did it fail? Of course, we are all grateful that we have to ask the second question.

Rube Goldberg device
The device has been called a "Rube Goldberg contraption" after a long-deceased American cartoonist, famous for his depictions of complex devices that perform simple tasks in indirect, convoluted ways. The Shahzad VBIED contained gasoline, propane, firecrackers, alarm clocks, a metal pressure cooker, fertilizer packed in a gun case, and what was described as a "bird’s nest of wiring." The widespread view among investigators and bomb technicians is that the bomb was of such a primitive and amateurish design that it was very unlikely that it ever would have exploded.

How was it supposed to work? According to NY City Police Commissioner Kelly, "it was believed that the timers would ignite the can of explosives, and that would cause the five-gallon cans to go on fire and then explode the propane tanks and have some effect on that rifle box."

Investigators believed that the fuses on the firecrackers had been lighted, but they did not explode, officials said. Apparently a portion of the Nissan Pathfinder’s interior ignited, causing a small fire that filled the inside with smoke, alerting nearby civilians who notified police. The detonating mechanism simply lacked the energy needed to properly set off the explosion, according to a former agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

There were a number of failures, largely in the selection of components. The triggering mechanism was identified as a can filled with about two dozen M-88 type commercial firecrackers. M-88s are sold legally in some states and Shahzad was captured on surveillance video buying fireworks from a company in Matamoras, Pennsylvania.

Fireworks and Gasoline
The M88 has just a tiny fraction of the explosive content of an M80 cherry bomb, now illegal to sell in the U.S. One report estimates that a stick of dynamite has thousands of times more explosive power than an M88. Further, firecrackers have explosive content designed to burn very quickly with a flash and bang, so lack the power of dynamite or other commercial explosives. According to Bruce Zoldan, president of the company that sold Shahzad the fireworks, "the M-88 he used wouldn't damage a watermelon."

The SUV also had two red plastic five-gallon containers of gasoline. From photos these appear to be regular safety cans, which if capped would have contained the potentially dangerous flammable vapors. Left uncapped for sufficient time, the vapor and air mixture might very well have been ignited by the M-88s. With sufficient heat generated, the plastic could melt and the containers could fail, creating a larger fire, leading to other complications. But given the location, the expected prompt response of the FDNY would have suppressed such a fire with little damage likely except to the vehicle.

Propane and fertilizer
Next to the gas containers were three propane cylinders, similar to those used for barbecue grills. We have all heard about BLEVEs, boiling liquid expanding vapor explosions, caused by fire impinging upon propane tanks. However, today’s tanks are very strong and well designed. When exposed to fire, a pressure relief device functions, and while this releases flammable gas, it prevents the cylinder from catastrophic failure. Again, there would have been fire, but a prompt response would have suppressed the fire before the cylinders reached the failure point.

Under no circumstance would the M-88s have penetrated the cylinders and detonated the gas. I have seen videotape of efforts to BLEVE propane cylinders for demonstration purposes, and it is clear that it is almost impossible to do.

And finally there as the question of the fertilizer, eight bags packed inside a 55-inch-tall metal gun locker. Later it was determined that it was a non-explosive grade of fertilizer, not the ammonium nitrate used in past terrorist bombs and by Oklahoma City bomber Timothy McVeigh. The wires from battery-powered fluorescent clocks ran into the gun locker, where a metal pressure-cooker pot contained a thicket of wires and more M-88s, according to Commissioner Kelly.

Even the right kind of fertilizer would have had to be mixed with the right proportion of fuel oil, in order the create ANFO, the preferred blasting agent used by professional blasters. Then, since ANFO is very insensitive, it requires a detonator and a booster. Then it's difficult to achieve a dense-enough packing of fertilizer so that the initial explosion doesn't just disperse the rest of the fertilizer into a cloud of dust, Thus, I assume, the metal rifle box.

London Bombing Attempt
The construction of the Times Square bomb is similar in design to the VBIEDs used in the attempted attack in a London neighborhood of nightclubs and theaters in 2007. In that case, the bombers had opened the propane cylinders to release the gas prior to detonation, but the bomb failed because the fuel-air mixture inside the vehicle had become too rich to burn. Following that attack, there was concern voiced by counterterrorism officials that such bombs could be used in the United States, especially in areas of high pedestrian traffic, such as in New York City.

Since 9/11, counterterrorism professionals have warned that it is not a matter of if, but when. In the past year, the Fort Hood shooter, the failed underpants bomber, and now Shahzad indicate that extremists still have America in their crosshairs. Terrorists will keep coming at us for the foreseeable future. VBIEDs have created many fatalities and tremendous devastation in Iraq and Afghanistan and are the terrorist weapon of choice. We must continue to learn and be ready for the next attempt.

Gordon Massingham is the Director of the Emergency Film Group’s program IEDS and VBIEDs, and has been creating counterterrorism programs since 1996.